feat(autoindent): On-Type auto-indent

[bryphe]

Issue: The way we were modelling decreaseIndent was incorrect - it's done on-type (on return, in some editors)

Fix: Implement an on-type auto-indent provider that we can hook in our decreaseIndent pattern / closing bracket patterns.

• Test with go, which has decrease/increase indent expressions
• Test with ruby (def/end): https://discord.com/channels/417774914645262338/417774914645262341/832759527844741182
• Stability test
• Fix backspace case
• Fix another backspace case (goes from 'Keep' -> Decrease) when backspacing