Debt: Open File should be in core, not the git extension

[hediet]

The command "Open File" is not implemented in the core, but in the git extension 🤯 (even when comparing two files on disk)

I don't see any reason why it couldn't be in the core. This would prevent bugs such as #196084.

https://github.com/microsoft/vscode/blob/cdb1a962ccd40d6d03132e7974a835c98f6e82e6/extensions/git/src/commands.ts#L1163

[lszomoru]

Adding @joaomoreno to see if there is any context that we are missing.