editor: Extract fold, selection, coade_actions and completions out of editor.rs

[GoldStrikeArch]

cc @SomeoneToIgnore

Summary

Follow-up to #55352, where the conclusion was to split editor.rs incrementally by topic instead of all at once.

This extracts several editor topics into focused sibling modules:

• crates/editor/src/fold.rs
• crates/editor/src/code_actions.rs
• crates/editor/src/completions.rs
• crates/editor/src/selection.rs

While doing the extraction, this also tightens the module boundaries instead of preserving every old access path by inertia:

• Avoids exposing the raw context_menu storage and replaces external usage with focused APIs for code actions and completions state.
• Keeps broad editor editing/cancel behavior in editor.rs, since those APIs are not selection-specific.
• Moves linked-editing range lookup into linked_editing_ranges.rs, closer to its only real caller.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• N/A

[SomeoneToIgnore]

Hard "nope", sorry.

One of the points is to keep things mechanical enough to review quickly and this is not the case now.
Also, why there's so many + in that particular file, seems odd?

[GoldStrikeArch]

Also, why there's so many + in that particular file, seems odd?

Yeah, this is weird. It should be -4k lines. I guess there is some re-ordering I introduced. I will try to fix it

[GoldStrikeArch]

@SomeoneToIgnore

Looks like something is wrong with the diffing here from the Github/git because on my branch git diff --minimal showed 12 insertions(+), 4610 deletions(-)

I can't solve it for some reason, so I decided to break them up into 2 sequential PRs, this is the first one -> #56030