Support for templ

**What problem does this solve?**
Hey there, I would love support for [templ](https://github.com/a-h/templ). Templ is a templating language for Go that sites on top of HTML.

**Proposed Solution**
I'm not sure how Harper works under the hood, but there is an (unofficial) tree-sitter implementation [here](https://github.com/vrischmann/tree-sitter-templ). I don't know if that helps with speeding up implementation?

**Examples**
N/A

**Component**
- [x] Core engine
- [ ] Plugin/Extension
- [ ] Other: _____

**Additional Context**
N/A

Start by reviewing Harper's core engine and the linked unofficial tree-sitter-templ implementation to determine how templ syntax could be supported. Done means Harper provides grammar checking support for templ files; no specific files or tests are identified in the issue.

Written by the indexing model from the issue text.