Support taking options from a file

(This is a feature request.)

Bindgen has a lot of parameters that take regexes. --allowlist-file, in particular, takes a regex with file paths. Because bindgen seems to not properly normalize / and \ on Windows, and instead punts this basic cross-platform operation to the user, those regexes get quite complex. Escaping and quoting such things across build systems and platforms is, sadly, a nightmare. (Many build tools pass their inputs through the shell. It's a huge mess.)

Additionally, projects that need to support multiple build systems have to repeat parameters in multiple ways. Right now, a project's expected bindgen flags needs to be repeated in every build sytem.

bindgen should support some kind of config file that allows passing these options in a more defined manner.

No files or tests are named in the issue. Start by locating bindgen’s command-line option entry point and how --allowlist-file is parsed, then review the existing option handling before defining a configuration-file approach. Done should allow projects to express bindgen options without repeating shell-dependent arguments across build systems and platforms.