Re use readonly #12732 #13863

[Ikuyadeu]

Reopen #13863 and will Solve #12732

• all in env (https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.d.ts#L3379)
• rootPath in workspace (https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.d.ts#L3870)
• textDocuments in workspace (https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.d.ts#L3921)
  are not. because they're using export and namespace.

[jrieken]

Thanks. Merging - we loose having explicitly spelled out readonly in the generated doc page because typedoc doesn't preserve that information. Still, the benefit outweighs that.