Convert xrdb to VS Code format

[jdhmtl]

There is an existing Powershell script that does this, but I updated the Python script (which was actually a copy of the Windows Terminal converter) so as to be easily callable from update_all.py. Closes #307

[jdhmtl]

I have taken care to minimize the impact on existing VS Code schemes, mostly to prevent having a giant PR to review. The current implementation does remove the trailing comma from the resultant JSON output, though, so a subsequent PR will add missing VS Code themes, and have a bunch of changed files where the only change is the dropped trailing comma. I know that's a little annoying, which is why I explicitly haven't included the updated themes here.

[mbadolato]

Thanks!