have custom file types treated as markdown notes

### 🚀 The feature, motivation and pitch

I like to keep my leet code exercises as obsidian notes. We can markdown style comment blocks in a cpp file, the only issue is that Obsidian does not recognize a cpp extension as a note file, can we have the option to enable some custom file extensions (or patterns that match the file name) to be treated as a plain markdown note?

### Alternatives

_No response_

### Additional context

_No response_

The issue names no files, tests, or entry points. Start by locating where obsidian.nvim identifies note files and applies markdown handling; done means configurable extensions or filename patterns are treated as plain markdown notes without changing existing note behavior.

Written by the indexing model from the issue text.