Add a CLI command to list info about built-in and discovered languages and themes

[andrewbranch]

Many of the built-in languages can be referenced by several names. But this info is extracted from grammar files within the vscode submodule, so it’s hard to point people to a place where they can see all recognized languages and how to reference them. Plus, it might change slightly in new major versions if I update the vscode submodule. The same command would also be useful for validating that third-party extensions were discovered correctly.