Standardize LSIF capabilities vertex

[gundermanc]

C# and TypeScript LSIF generators both have support for capabilities vertex. This doesn't yet appear to be referenced in the spec, however.

Such capabilities nodes look like:

{
  "hoverProvider": true,
  "declarationProvider": false,
  "definitionProvider": true,
  "referencesProvider": true,
  "typeDefinitionProvider": false,
  "documentSymbolProvider": false,
  "foldingRangeProvider": true,
  "diagnosticProvider": false,
  "semanticTokensProvider": {
    "tokenTypes": [
      "namespace",
      "type",
      "class",
      "enum",
      "interface",
      "struct"
    ],
    "tokenModifiers": [
      "static"
    ]
  },
  "id": 1,
  "type": "vertex",
  "label": "capabilities"
}

This is a requirement for #1754 which relies on the tokenTypes and tokenModifiers arrays above to decode encoded semantic tokens into token type names.

Open Questions:

• Is this entry a vertex or not - In C#, it's not explicitly declared, in TypeScript it is.
• Where in the file is it - In C# it's the first line. In TypeScript it tends to be 2nd or 3rd. If not first, what is the criteria for finding it, what if anything can come before it, and what is the expected consumer behavior when it is absent?

[dbaeumer]

It is a vertex and it describes which capabilities (in terms of LSP capabilities the dump supports). This allows LSP servers that surface a LSIF graph to provide the right capabilities on the intialize response.

The constraint I would add to the spec is that the vertex needs to be emitted BEFORE that first project vertex is emitted.