[compiler][snap] Support pattern of files to test as CLI argument

I've been trying out LLM agents for compiler development, and one thing i found is that the agent naturally wants to run `yarn snap <pattern>` to test a specific fixture, and I want to be able to tell it (directly or in rules/skills) to do this in order to get the debug output from all the compiler passes. Agents can figure out our current testfilter.txt file system but that's just tedious. So here we add support for `yarn snap -p <pattern>`. If you pass in a pattern with an extension, we target that extension specifically. If you pass in a .expect.md file, we look at that specific fixture. And if the pattern doesn't have extensions, we search for `<pattern>{.js,.jsx,.ts,.tsx}`. When patterns are enabled we automatically log as in debug mode (if there is a single match), and disable watch mode.

Open to feedback!