Add new button configuration schema

[atakiya]

Basically a revert of the revert of d689dfb35d5fb180d53ebfe2cfe7d64701d1ef1f, but now supporting autocomplete and validation for both types of button configurations, among a couple fixes here and there. Most, if not all of the values have been copied from the pre-existing ones.

Closes #195

Note: This is probably not ideal

1. normal JSON schemas allow using $ref to minimize repetition, which is supposedly not supported in vscode extensions manifests, so a lot of the schema is repetitive (see https://code.visualstudio.com/api/references/contribution-points#Configuration-property-schema) - this makes maintenance in my opinion worse.
2. Descriptions for most of the entries are missing, I'm not entirely sure how the buttons activate in regards to the git and non-git buttons, so I've left that out for now.
3. I'm unaware of how the extension prioritizes the configuration. This is probably quite important to communicate to endusers, because one of them most likely overwrites the other and can cause confusion.
4. Advanced schema tables like this do not support showing an UI in the vscode settings

[atakiya]

Maybe tho change the config back to what it default was tho xD

Could you elaborate? The schema defaults have been set to the same values that the other/old configuration options are set to.

[nick22985]

yea I think it has been changed previously and I didn't notice it lol. This is the new default dw sorry mb.