[Wildcard Variables] CFE Implementation

[kallentu]

Issue may be edited as we add more items of work.

A general outline of areas that need to be updated for the wildcards feature:

• Decide how we would like to represent _ in the CFE (body_builder)
• Update function signatures allowing multiple _s (outline_builder)
• Update the scope look up to avoid finding local declarations of _. May come naturally if we don't register _ as valid in-scope elements. (body_builder)
• Update error handling.
• Suppress unused variable warnings for _ local declarations.
• Add unit tests and CFE file tests.

cc. @chloestefantsova @johnniwinther @stereotype441