Agent: implement `createFileSystemWatcher` shim

[valerybugakov]

• Required for vscodeGitAPI.onDidOpenRepository and vscode.workspace.onDidChangeWorkspaceFolders.
• Multiple existing product features (e.g. Repo Picker) depend on these APIs while they are not supported in clients other than VS Code.
• https://github.com/sourcegraph/cody/blob/c49642c33a28564c0476a4c41dd1d66f112c331d/agent/src/vscode-shim.ts#L349

[olafurpg]

File watching is very expensive to operate in large repositories, we should lean heavily on the editor’s existing file watchers over rolling our own.

[github-actions[bot]]

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