Add support for VSCodium

[helgi]

VSCodium (https://vscodium.com/) is a community-driven, freely-licensed binary distribution of Microsoft’s editor VS Code.

The way extensions are exported and imported is the same as VSCode; There was a consideration to add version support (codium --list-extensions --show-versions) but it is not required as the IDE will update extensions itself

[jacobbednarz]

i appreciate the effort here however, i'm a 👎 on the amount of duplication we have here for the multiple flavours of vscode. i would much rather see us provide an additional argument to target the binary/path for the various flavours of vscode or, the much bigger ask, generalise the plugin API so that all extensions can be extracted from core all together and push this to third party providers.