Fix getAsyncAndAwaitOccurrences function handling

[jakebailey]

Fixes #1836

This is just a pure port. Seemingly no tests hit this.

[Copilot]

Pull Request Overview

This PR standardizes the handling of function lookups by moving the getContainingFunction utility from the checker package to the AST package as GetContainingFunction and updating all references to use the centralized version.

• Moves getContainingFunction from checker utilities to AST utilities as a public function
• Updates all call sites across the codebase to use the new AST package function
• Removes the duplicate implementation from the checker package

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/ast/utilities.go	Adds the new GetContainingFunction function to the AST package
internal/checker/utilities.go	Removes the old getContainingFunction function
internal/checker/grammarchecks.go	Updates call site to use ast.GetContainingFunction
internal/checker/checker.go	Updates multiple call sites to use ast.GetContainingFunction
internal/ls/documenthighlights.go	Updates call site and refactors variable names for consistency