Move no longer needed extensions to separate category, update links

[krassowski]

• git extension is no longer in alpha
• removed "VS Code" reference, this is a trademark and while LSP and VS Code share LSP servers the implementations of frontend are completely unrelated
• updated spellchecker URL
• removed go to definition, I no longer maintain this and users should install LSP extension instead.
• moved extensions that should no longer be installed because they are in core to a seperate category.