fix(experimental elaborator): Don't check return type for trait methods

[jfecher]

Description

Problem*

Resolves some errors in the elaborator from checking a trait method's empty body against the expected type that was declared.

Summary*

Adds whether or not a function is a trait method (not a trait impl method) to the is_stub method which is just a renamed can_ignore_return_type.

Additional Context

Down to 203 errors in the stdlib.

This also fixes unused variable warnings that were present before but not shown in the error count.

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.