pgs _headers file behavior description

[jl]

It would be nice if the documentation here:

docs/posts/pgs.md

Line 325 in 7397057

We support custom headers via a special file `_headers`.

was more specific about the behavior which occurs here:

https://github.com/picosh/pico/blob/96bba54c241d82ae4916b3253f404d062490d0ff/pkg/apps/pgs/web_asset_handler.go#L252

That is:

• go regular expressions are supported, with the caveat that the first character must be '/'
• the match succeeds anywhere in the path, and it is not possible to match only the beginning of the path, since an expression starting with ^ will not be recognized as a valid rule; see isPathLine
• all header rules are checked in order and the last match "wins", headers do not accumulate, rules are not combined
• the example /* might be more clearly commented if the intention is truly "zero or more slashes anywhere in path"

Here's a rule set that could help readers understand:

# generic rule must be listed first, since last match wins;
# note if /cross/ appears anywhere, even in the middle of the path,
# the match will succeed.
/cross/
  Access-Control-Allow-Origin: https://user-othersite.pgs.sh

# serve files ending in .mjs as text/javascript instead of text/plain
/cross/.*\.mjs$
  Access-Control-Allow-Origin: https://user-othersite.pgs.sh
  Content-Type: text/javascript