vscodium: fix symbolic linking issue with executable

[Tbusk]

Summary

Currently, the symlink between the executable and the bin file to run it isn't working. It silently fails to create the symlink. Chatted in matrix and found a solution, which this implements. Also, this adjusted the value of mime type to match flatpak version just in case it causes an issue with some part of the app I did not test that expects it.

Test Plan

• Cleared existing symlinks and installed vscodium files and reinstalled. Shows up in app menu and launches correctly. Files show up in correct location as well.

Checklist

• Package was built and tested against unstable
• This change could gainfully be listed in the weekly sync notes once merged
• I agree to license this contribution and all my previous contributions under the licensing terms in LICENSE.md and have the power and authority to grant those licenses.

[EbonJaeger]

Could this last sed be avoided by doing the desktop-file-edit after building/installing?

[Tbusk]

From what I can tell, upstream fills in the values only for like snaps and deb builds for linux here: https://github.com/microsoft/vscode/blob/main/build/gulpfile.vscode.linux.ts

So the replacement needs to be done manually. The reason I had to use sed was because desktop-file-edit only touches stuff in the [Desktop Entry] section, not the [Desktop Action ... ] section, which left icon and exec in that area untouched.

https://github.com/VSCodium/vscodium/blob/master/src/stable/resources/linux/code.desktop

I think a cleaner approach might be to create a patch or recreate the .desktop file and throw it in with the files.