Migrate from `vscode-nls/vscode-nls-dev` to `vscode-l10n`

[gcampbell-msft]

The underlying libraries that facilitate string localization in VS Code extensions (vscode-nls/vscode-nls-dev) have been replaced with a new library: vscode-l10n.

https://github.com/microsoft/vscode-nls
https://github.com/microsoft/vscode-nls-dev
https://github.com/microsoft/vscode-l10n

Opening this issue to track migrating to the new library.