Manage ResourceWarnings

[willmcgugan]

We run Textual with ResourceWarnings enabled. We should disable these by default, but enable them via textual run --dev.

The shouldn't write to stdout or stderr, which may break the display. They can go to dev tools, and also stdout on exit.

[darrenburns]

Warnings write to stderr by default, and we already capture them and send them to the devtools. If --dev isn't enabled, then we swallow them silently.

We'll park this one for now and see if it comes up in the future.