fix: Handle malformed setting for user languages for CS

When the user language setting for code suggestions was malformed, parsing it prevented the initialization of the extension. We change the type of the setting from string to string[] and check that the array was provided.

To test it, set the incorrect value in the settings.json and make sure the extension is starting and code suggestions for the default languages are still working as expected.

Example of malformed input value:

  "gitlab.aiAssistedCodeSuggestions.additionalLanguages": false

This functionality should work both with the LS enabled and disabled and is depended on this LS MR gitlab-org/editor-extensions/gitlab-lsp!315 (merged)

Closes #1358 (closed)