Parse fun in lambdas and fun in function references as different tokens

[kvakvs]

Fixes #990

Description

• Split BNF definitions for lambda fun() -> ...; () -> ... end, and fun mod:func/arity reference (just for clarity)
• Split ERL_FUN="fun" token into two: "fun" followed by a "(" for regular function definitions, and ERL_FUN2 = "fun" followed by a letter for function reference expressions: fun mod:func/arity.
• Modify the highlighting to color ERL_FUN2 as a keyword
• Ensure parser, highlighter and formatter tests handle this change well

TODO Checklist

• PSI/parser integrity tests failing
• Inline variable test failing
• Formatting tests
• SDK-specific tests failing on Windows (retest on Linux or Mac)

Demo

Cursor standing in the inner fun is not showing as paired with end

Cursor in outer fun is paired with end