[WIP] Introduce new code action: suggest missing fields

[akshaymankar]

Detects -Wmissing-fields warnings and suggests a code action to add all the missing fields.

Resolves #462

Pending tasks:

• [ ] The basic test is failing because there are extra new lines
• [ ] Add cases for HsProc
• [ ] De-duplicate some copy-pasted code
• [ ] Extract the code action into its own module
• [ ] Add a few more test cases

[akshaymankar]

The basic test is failing because there are extra new lines

I need help with this one. I tried adding traceAst everywhere, but it still doesn't make sense to me.