Chore: split monster-sized modules in the codebase

[psfinaki]

We have a bunch of modules that are so big that they cause code editors to hang. The champions are probably in the compiler project, for example CheckExpressions (>10,000 LOC) or CheckDeclarations (>5,000 LOC).

We should take some monster every sprint and cut it into parts.

[dsyme]

See comment here #13652 (comment) about future ways to split more