Extract LineIndex independent methods from Locator

[MichaReiser]

Summary

The Locator provides useful methods to extract a line's range or text given an offset
or the full range or text of all lines falling into a text range.

These methods are foundational and used across many crates.

However, Locator also exposes methods that require computing and memoizing a LineIndex.
The formatter doesn't use any of those methods and "cheats" by always creating an ad-hoc Locator
instead of storing an instance on PyFormatContext. This leaks the Locator abstraction
because it depends on the knowledge of which Locator methods require the extra LineIndex state.

This PR splits the "stateless" methods from Locator into a new trait and implements that trait
for str (string slices). It also replaces all Locator usages with &str in all crates other than ruff_linter.
It also moves Locator to ruff_linter to clarify that this is linter-specific infrastructure.

I decided to keep Locator for ruff_linter because:

• Locator communicates that this is the source of the entire file
• Some linter code uses the LineIndex dependent Locator methods where the lazy index computation is useful.

We could consider changing more Linter methods to take &str instead of Locator but we can do this in separate PRs.

Renamed methods

I had to rename lines to lines_str str.lines conflicts with the std lines builtin (returns an iterator over the lines). I decided to add the _str postfix to all other methods that return a str for consistency.

Test Plan

cargo test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[AlexWaygood]

Nice, this is great!