Support _ as the first character in environment variables

[paul-english]

This updates the validation regex to allow keys in an env file to start with an underscore.

I just noticed this in conflict with an env var _TYPER_STANDARD_TRACEBACK. This SO post seems to have a pretty valid answer sourcing this document about this https://pubs.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap08.html

[willkg]

This looks good. Can you please add a test for it? If you can't, I'll get to it in the next month or so. Thank you!

[willkg]

Also, it's helpful when the commits have useful summaries. For example, this one would probably be summarized as "Support _ as the first character in environment variables". Can you fix that as well?

[paul-english]

Sure, I'll double check it and see where to add a test. This edit was from the GH interface, thus the brevity.

[willkg]

I find the GitHub interface is great for docs fixes, but harder for code fixes where multiple files need to be changed. In that case, I tend to write up an issue instead. If you want, convert this to an issue and I can work through it when I get to it.