Add 'Extract tuple into struct' code action

[krishnapermi]

Implements the "Extract tuple into struct" code action requested in #2511.

When the cursor is on a named tuple type — one where every element has a label — a new code action appears in the lightbulb menu. It generates a struct with matching stored properties immediately before the enclosing declaration and replaces the tuple type annotation with the new struct name.

Before:

func getUser() -> (name: String, age: Int, email: String) {
    return ("Alice", 30, "alice@example.com")
}

After:

struct GetUserResult {
    let name: String
    let age: Int
    let email: String
}

func getUser() -> GetUserResult {
    return ("Alice", 30, "alice@example.com")
}

The struct name is inferred from context:

• Function return type → <FunctionName>Result (e.g. getUser → GetUserResult)
• Variable annotation → capitalised variable name
• Type alias → the alias name itself
• Fallback → concatenated capitalised element labels

Indentation is matched to the surrounding source using BasicFormat.inferIndentation.

Scope: This is the syntactic-only step discussed in #2511 — only the local type annotation is rewritten. Renaming call sites that construct or destructure the tuple is left to a follow-up rename refactoring (the pattern @ahoppen described as an acceptable first step).

What doesn't trigger it: unlabelled tuples, single-element tuples, and tuples with _ wildcard labels, since none of those produce clean property names.

Resolves #2511.

[ahoppen]

Some initial comments. Could you also add test cases for this?

[ahoppen]

Could you implement this in terms of SyntaxRefactoringCodeActionProvider?

[ahoppen]

I don’t think this comment provides significant value as it just describes what the code below already clearly states. Please remove such comments.

[ahoppen]

This would read nicer as a multi-line string literal

[ahoppen]

Can’t you use findParentOfSelf here instead of walking the tree manually?