Textmate scope selector: support `>` combinator (child)

Summary

Support the child operator/combinator for scopes in Textmate scope selectors.

path: '^'? «scope» ('>'? «scope»)* '$'?

https://macromates.com/textmate/manual/references#scope-selector

Motivation

The child combinator is used to restrict scope match in path to immediate children, much like > in CSS.

Describe alternatives you've considered

Being specific enough in the grammar and selector to target immediate children, but that assumes:
a. control over the grammar and
b. a scope selector that's amenable to this.

Additional context

Might be adopting these helpers for MSFT engineers in the VS Code ecosystem?