Update extensionPaths when web extension host started

[thegecko]

When in web-server mode, there are a few issues loading extensions from the backend which run in the web extensionHost when they offer task providers or debug providers. e.g.:

• Web extension host debug adapters not working with codespaces/github.dev #171991
• Provided tasks are not available when VSIX extension targets web #159557

Debugging this has shown this is due to a mismatch in extension IDs here (the loaded extension ID begins with nullExtensionDescription) and lack of registered debuggers found here.

These issues are both due to a race condition where the extensionPaths initially used for loading fake extensions in the web extensionHost don't get updated when the host is started properly.

The flow seems to be:

• extensionPaths are loaded when using fake extensions and injected into the interceptors:

  vscode/src/vs/workbench/api/common/extHostRequireInterceptor.ts

  Line 60 in eec8448

  const extensionPaths = await this._extHostExtensionService.getExtensionPathIndex();

• the web host is started and extensions updated:

  vscode/src/vs/workbench/api/common/extHostExtensionService.ts

  Line 986 in eec8448

  this._myRegistry.set(myExtensions);

• the interceptors are loaded using the initial extensionPaths (which are invalid):

  vscode/src/vs/workbench/api/common/extHostRequireInterceptor.ts

  Line 165 in eec8448

  const ext = this._extensionPaths.findSubstr(parent);

• default extensions are created in the web host:

  vscode/src/vs/workbench/api/common/extHostRequireInterceptor.ts

  Line 176 in eec8448

  if (!this._defaultApiImpl) {

• many errors occur

This PR adds an event for when the extensions are reloaded during the host start. This event is used to re-inject the updated extensionPaths. I'm pretty sure there's a better approach, so would be grateful of some support chaperoning this bug fix.

[thegecko]

Please let me know if you need help reproducing this bug

[alexdima]

Thank you!