Fix sort_requires not recognizing const imports in luau

[ddashdev]

Problem

sort_requires only recognized imports parsed as Stmt::LocalAssignment, so Luau imports declared with the new const keyword were ignored.

For example, this stayed unsorted:

const Zebra = require("Zebra")
const Apple = require("Apple")

Summary

• Adds Luau-only handling for Stmt::ConstAssignment in sort-requires
• Reuses the same require-matching logic for local and const assignments
• Preserves existing grouping, blank-line splitting, ignored-node behavior, and game:GetService sorting
• Adds tests for const requires, mixed local/const blocks, type assertions, and services
• Updates docs and changelog