Watch mode will not be settable from command-line after npm 12 flag parsing changes

According to https://github.com/npm/cli/pull/8071 (which adds a warning), npm will change the way arguments are parsed in a way that will break the way wireit looks for the `--watch` environment variable. Previously, `npm run build --foo` would set the `npm_config_foo` environment variable, and we relied upon that fact for detecting when the `--watch` flag had been sent. In npm 12, this will be an error about "watch" being an unrecognized npm argument. This seems like a good change for npm (silently eating arguments does seem bad), but I don't believe we currently have another way to enable wireit watch mode. Proposals: 1. Add a `WIREIT_WATCH=1` environment variable. 2. Special case `--wireit-watch` (and perhaps reserve `--wireit-` prefixed flag) to be extracted from the additional arguments that are usually passed down to the actual command (e.g. `npm run build -- --wireit-watch --flag-for-build`) 3. Solve for https://github.com/google/wireit/issues/553 to allow a script to be configured as intrinsically watching (e.g. a `{"watch":true}` config setting), so that people can easily define e.g. `build:watch` scripts instead of using flags in the first place. I'm leaning towards doing 1 in the short term, because we already handle all other runtime wireit settings with `WIREIT_` environment variables and it's very easy, and 3 as a followup.