Implement more handlers

[renkun-ken]

This PR implements the following handlers:

• prepareTypeHierarchy
• typeHierarchySupertypes
• typeHierarchySubtypes
• semanticTokens

and makes the following improvements:

• activeParameters in completion is supported.

• hierarchicalDocumentSymbolSupport is implemented so that class members are displayed in outline.

• The range of top-level symbols are fixed so that symbols defined in global scopes (enclosed in {} with or without if while etc) have correct ranges.
• Add parse cache and diagnostics cache to reuse existing results of parsed document to be more responsive.
• Rewrite semantic token extraction and code point calculation in C to reduce R allocation for better performance.
• Add arg_value_completion so that completion items for the arg values in function like fun(arg = c("value1", "value2", etc)) are supported.

P.S: The code written in this PR is mostly done by @copilot.

[Copilot]

Pull request overview

This PR expands the R language server’s LSP feature set (type hierarchy + semantic tokens) and improves responsiveness by adding caching and moving several hot-path operations to C.

Changes:

• Implement textDocument/prepareTypeHierarchy, typeHierarchy/supertypes, and typeHierarchy/subtypes with supporting class/member extraction.
• Add textDocument/semanticTokens/full and /range plus server capability wiring.
• Improve performance via parse/diagnostics caching, task scheduling tweaks, and new C implementations for token scanning, UTF-16/code-point conversion, semantic token encoding, and matching.

Reviewed changes

Copilot reviewed 43 out of 45 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tests/testthat/test-type-hierarchy.R	Adds type hierarchy tests (R6).
tests/testthat/test-symbol.R	Adapts symbol range assertions via helper.
tests/testthat/test-symbol-blocks.R	Tests symbols in top-level blocks.
tests/testthat/test-signature.R	Adds activeParameter test cases.
tests/testthat/test-semantic-tokens.R	Adds semantic tokens tests (full/range).
tests/testthat/test-lintr.R	Stabilizes lintr tests/config handling.
tests/testthat/test-hover.R	Makes hover tests less brittle.
tests/testthat/test-highlight-blocks.R	Tests highlights in top-level blocks.
tests/testthat/test-definition-blocks.R	Tests definition in top-level blocks.
tests/testthat/helper-utils.R	Adds request helpers + tweaks test server startup.
src/token.h	Declares C token scanner.
src/token.c	Implements fast token scanning in C.
src/semantic.h	Declares semantic token encoder.
src/semantic.c	Implements semantic token delta encoding in C.
src/match.h	Declares C matching helpers.
src/match.c	Implements ASCII case-insensitive/fuzzy matching.
src/encoding.h	Declares UTF-16/code-point conversion helpers.
src/encoding.c	Implements UTF-16/code-point conversion in C.
src/languageserver.c	Registers new .Call entry points.
README.md	Marks new LSP features as implemented.
R/workspace.R	Adds parse/diagnostics caches + cache cleanup.
R/utils.R	Adds ASCII detection, C-backed match/fuzzy, C token scan wrapper, and C-backed UTF-16 conversions.
R/type_hierarchy.R	Implements type hierarchy logic and class member extraction.
R/task.R	Adjusts task scheduling/utilization for parse tasks.
R/symbol.R	Adds hierarchical DocumentSymbol support + class member children.
R/signature.R	Adds activeParameter computation and parameter range parsing.
R/settings.R	Introduces diagnostics cache TTL setting.
R/semantic.R	Adds semantic tokens extraction + encoding + handlers.
R/selection.R	Formatting/indentation adjustments only.
R/rename.R	Formatting/indentation adjustments only.
R/references.R	Refactors reference collection to avoid repeated c() growth.
R/languageserver.R	Rescales session pool sizes and adds pending reply queues for new requests.
R/interfaces.R	Adds document_symbol() constructor for hierarchical symbols.
R/hover.R	Updates XPath roots to include more node types.
R/handlers-textsync.R	Parses immediately on open (delay=0).
R/handlers-langfeatures.R	Adds handlers for type hierarchy + semantic tokens requests.
R/formatting.R	Avoids repeated list growth when building edits.
R/document.R	Fixes srcref propagation for block parsing + adds parse caching by content hash.
R/diagnostics.R	Adds diagnostics caching + explicit linters fallback logic.
R/definition.R	Updates XPath roots similarly to hover/signature.
R/completion.R	Avoids repeated list growth + updates XPath roots.
R/code_action.R	Avoids repeated list growth when collecting actions.
R/capabilities.R	Advertises type hierarchy + semantic tokens capabilities.
NAMESPACE	Imports digest::digest.
DESCRIPTION	Adds digest dependency + updates RoxygenNote.

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