feat(resolve): optimize applying overrides to be efficient

This PR addresses the inefficiency of processing `# gazelle:resolve` directives. Previously, the system iterated over a slice for each import to find matching directives, which is very inefficient. This PR introduces two major improvements:

1. **Transition from Slice to Map**: We've shifted from using a slice to a map for storing override configurations. This change reduces lookup time complexity from O(n) to O(1) (or O(length of chain of enclosing dirs containing resolves)). 

2. **Parent-Child Config Structure**: To maintain directory-specific configurations without the overhead of copying maps (which is very inefficient), we've implemented a parent-child relationship in configurations. This structure allows each directory to have its unique overrides while inheriting unmodified settings from its parent. We can't use a single shared map, because this could mistakenly apply directives from one directory to another. `regexp` overrides are always copied and evaluated inline. 

Note on GC: to avoid a really complex object tree, the parent/child relationship skips all ancestors without `# gazelle:resolve` directives, greatly reducing the complexity.

Note of Regexp: although we need to iterate through these due to the matching logic, we could at least have a `map: lang -> []overrides` to improve the lookup some.

Fixes #1688