feat: support ellipsis in ruby patterns

[didroe]

Description

Adds support for $<...> in Ruby patterns. This suppresses anchors being omitted for nodes in the pattern that start/end at this position.

This is useful for cases such as matching arguments in any position, or when a method can have an optional block.

Also makes a pair always un-anchored as this seems more natural.

Closes #448

Checklist

• I've added test coverage that shows my fix or feature works as expected.
• I've updated or added documentation if required.
• I've included usage information in the description if CLI behavior was updated or added.
• PR title follows Conventional Commits format