[Headers] Add an umbrella header for the library

[ide]

Previous discussion in #261, looks like I accidentally closed it...

The umbrella header allows projects that link React Native to import it in standard fashion: #import <React/React.h>. Only some headers are included based on what I've found to be useful.

Generated with scripts/generate-umbrella-header.sh, which is a script that scans the Base and Executors directories for all headers and also includes some handpicked Views headers as well.

[nicklockwood]

Not a bad idea, but I'm concerned about the maintenance burden of keeping this up to date.

It's also probably not necessary to include most/any of the view and viewmanagers, as they're effectively private - they're only exposed to the JS code.

[ide]

@nicklockwood - i agree about the maintenance upkeep. I could make a little script that adds all the headers under Base and Executors, and then have a manually curated list of headers under Views (e.g. the categories).

[ide]

@nicklockwood - there is now a little bash script that generates the umbrella header that pulls in Base+Executors and some headers I handpicked from Views.

To reduce the chance of breakage (specifically removing/renaming a header and not removing it from the umbrella - although I imagine codemod/git grep would pick up the change anyway), I'll update the example projects to use the umbrella header if this looks good to you.

[sahrens]

over to nick to decide if we want to do this and review or close it out as something we don't want.