Add Wildcard Variables feature to the Language docs

[kallentu]

This issue tracks the work needed for wildcard variable docs.

Summary

Extends the wildcard behavior of patterns to more of the language, making local variables and parameters named _ non-binding.

Eval

• Add section to the Variables page
• Define wildcard variables:
  • non-binding, never in scope
  • new behavior applicable to top-level variables, parameters, (list all)
  • naming behavior doesn't affect top-level declarations (list all)
• Motivating examples:
  • anywhere you don't care about the value, just need a placeholder
  • annoying that _ binds a name and is therefore not reusable; removes the need for multiple underscore naming
• Any special cases/unexpected behavior for certain applications:
  • TBD (probably not)

Resources

• Issue
• Proposal
• Tracking issue
• Multiple underscore issue
• super._ issue