Add plaintext and directory mimetypes to Linux desktop entry

[Tech-Tac]

This allows users to open files and folders with VSCode directly from their file manager, matching the behavior of other text editing applications on Linux.

Fixes #15741

[rzhao271]

Likely needs to be tested across multiple distros, see #209510

[Tech-Tac]

As long as we are following the standard [1, 2], we should be fine.

[rzhao271]

I believe we were following those standards before, and then #114425 happened.
The issue is we want to register VS Code as an available application for those types but not accidentally become the default application for those types. Your second link seems to suggest a way to register a type as the default, which is the opposite of what we want. The ideal solution would be a post-installation command that establishes our application as not the default for some types.

[Tech-Tac]

There is not much we can do in any case, this sounds like bad behavior in the mentioned environment and nothing we can directly influence. We are simply following the spec.