Support for the Nim programming language

Nim is a newer programming language in the same space roughly as Zig, Odin, and Jai.

- [Nim's homepage](https://nim-lang.org/)
- [Nim Tree-Sitter grammar on GitHub](https://github.com/alaviss/tree-sitter-nim)

Start by reviewing Nim's homepage and the linked Nim Tree-sitter grammar to understand the requested language support. Then inspect how Harper currently supports programming languages; the issue names no files or tests, so define the integration points and acceptance criteria before implementation. Done means Nim support is integrated and verified by the project's relevant checks.

Written by the indexing model from the issue text.