Feat(SwiftRefactor): Implement CollapseNestedIfStmt refactoring

[rickhohler]

Description

This PR implements the CollapseNestedIfStmt refactoring action, which merges nested if statements into a single if statement with a combined condition list.

Transformation:

if a {
  if b {
    print("hello")
  }
}
// becomes:
if a, b {
  print("hello")
}

Detailed Design

• Adds CollapseNestedIfStmt provider to SwiftRefactor.
• Validation Logic:
  • Ensures outer if has exactly one statement (the inner if).
  • Ensures neither if has an else block (to avoid ambiguity).
  • Checks for IfExprSyntax inside the block.
• Transformation Logic:
  • Appends inner conditions to outer conditions.
  • Carefully handles comma insertion and whitespace trimming for the merged condition list.

Fixes

Addresses one part of swiftlang/sourcekit-lsp#2424 (Porting legacy refactoring actions).

Pre-PR Checklist

• Code builds and passes tests.
• Ran swift-format.
• Added unit tests in CollapseNestedIfStmtTest.swift.