Migrate from execa to tinyexec

[v1rtl]

identical API, 21 less dependencies

https://github.com/tinylibs/tinyexec

[davigiroux]

Started looking at this. The two APIs aren't fully identical though. tinyexec has no tagged-template form, no cleanup/reject options (defaults match), and the stream/event surface hangs off result.process.stdout/.stderr/.on(...) instead of the result itself.

prool's processes.execa uses the tagged-template form in its public callback:

process.start(($) => $`anvil --port 1337`, options)

Two ways I see to handle that before I PR:

1. Change the callback to (x) => x('anvil', ['--port', '1337']). Cleaner but breaking for downstream.
2. Wrap tinyexec in a small tagged-template adapter so the public surface stays put. Pulls in args-tokenizer (one dep), so net reduction is ~20 not 21.

Preference?

[v1rtl]

Second sounds better