Grammer checking in XML / other file types

I'm using Harper in VSCode and it is super for checking plain Markdown files - thank you! I also have a bunch of XML files which have Markdown in them (XML for the site structure and various structured properties, the CDATA sections in tags for the Markdown). I'm wondering if there is a way to configure what files Harper will apply to?

**What problem does this solve?**

It appears that Harper only performs checks on certain file types, and there doesn't appear to be a way to configure that.

**Proposed Solution**

I'd like to see an option that will allow the user to configure what file types it will run on - that could be on the file extension or the VSCode detected file type.

**Examples**

This image shows the same text, one in the XML file (right) and one in a plain Markdown file (left). Harper runs on the plain file, but not the content in the XML one.

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

Start with the Harper VSCode plugin/extension entry point that decides which file types are checked, and compare its handling of Markdown with XML files. Done means users can configure extensions or VSCode-detected file types so Harper checks Markdown embedded in XML or other selected types.

Written by the indexing model from the issue text.