Per-file banner

[arcanis]

It would be very useful if plugins were able to provide a set of custom defines or injects (basically a banner) on a per file basis without overriding the whole onLoad process (which would decrease perfs by preventing Esbuild from reading the files itself, and wouldn't work well with source maps).

It's kinda what you suggested in #859 for __dirname:

I think my preferred solution for this is going to be to use the --define feature in addition to adding the ability for plugins to add additional per-file configuration of input flags such as --define. This seems more general and appropriately minimal (not really adding more features, but just combining two existing features). This is not possible at the moment because plugins do not yet have the ability to configure these input flags per-file.