Allow underscore in number literals

Hi, it would be nice if number literals could contain an underscore for better readability when it comes to long numbers (e.g. `10_000`). Basically all major programming languages support this as well as some markup/configuration languages. If you agree that this is a good idea I could probably carve out some time to provide the implementation in the next couple of days.

The issue identifies number literals and the example 10_000 as the entry point; start by locating number-literal parsing and its existing tests in the HCL Go codebase. Completion should include accepting underscore-separated numeric literals while preserving existing numeric syntax, with tests covering the new form.

Written by the indexing model from the issue text.