[browser][coreclr] Fix corerun to run in browser again

[radekdoulik]

The corerun build uses -sEXPORT_ES6=1 -sMODULARIZE=1, producing corerun.js as an ES6 module. Several issues prevented it from running in the browser:

• corerun.html loaded corerun.js with a plain <script> tag, causing
  "Cannot use import.meta outside a module". Fixed by using

  <script type="module"> with an ES6 import of the selfRun export.

• libCorerun.extpost.js had 'var fetch = fetch || undefined' which
  in ES6 module scope shadows the global fetch API due to hoisting,
  breaking WASM loading. Removed the declaration. Also guarded
  process.exit() for Node.js only and made selfRun() auto-run only
  in Node.js (in browser it is called from corerun.html with config).

• The preRun callback used bare FS/ENV globals which don't exist in
  modularized output. Fixed by accessing them via the module parameter.
  Additionally, Emscripten's --preload-file data is loaded by an async
  preRun callback whose fetch hasn't completed when our preRun runs.
  Fixed by using addRunDependency/monitorRunDependencies to defer
  main() until the preloaded files are available in the virtual FS.

• libCorerun.js unconditionally depended on $NODEFS/$NODERAWFS which
  are unavailable in browser builds. Use LibraryManager.library checks
  to conditionally include them only when present.

Also added ENV to EXPORTED_RUNTIME_METHODS so the preRun callback can
set APP_ASSEMBLIES via module.ENV.

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[Copilot]

Pull request overview

Fixes the CoreCLR corerun WASM host so the ES6-modularized corerun.js can run in a browser again (in addition to Node), aligning corerun.html and the JS glue code with Emscripten’s -sEXPORT_ES6=1 -sMODULARIZE=1 output.

Changes:

• Update corerun.html to load corerun.js as an ES module and call the exported selfRun() with a module config.
• Adjust libCorerun.extpost.js to avoid shadowing global fetch, and to only auto-run / call process.exit() in Node.
• Add a browser-specific Emscripten JS library (libCorerun.browser.js) and export ENV so browser preRun can set APP_ASSEMBLIES.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/hosts/corerun/wasm/libCorerun.extpost.js	Makes selfRun() configurable and Node-only auto-run; avoids fetch shadowing in ES module scope.
src/coreclr/hosts/corerun/wasm/libCorerun.browser.js	Provides a browser-only Emscripten library variant without Node FS dependencies.
src/coreclr/hosts/corerun/wasm/corerun.html	Loads corerun.js as an ES module and invokes selfRun() with browser config + preload dependency gating.
src/coreclr/hosts/corerun/CMakeLists.txt	Switches JS library based on CORERUN_IN_BROWSER and exports ENV for runtime config.