vscode: add support for the l10n namespace

[msujew]

With vscode 1.73.0, a vscode native way has been introduced to localize extensions (which was previously done through the vscode-nls package). See their vscode.d.ts file. Microsoft now uses the API starting from 1.73.0 for all their built-in extensions, so we need to support/stub this in case someone wants to use current versions of built-in extensions.