feat: watch gen in `gro dev`

[ryanatkn]

See these PRs where this was previously implemented, and then removed with the build system.

• https://github.com/ryanatkn/gro/pull/283
• https://github.com/ryanatkn/gro/pull/382

Needs to be thought through in combination with Moss style optimizations.

We can't simply make a Vite plugin because gen modules and their dependencies may not be part of any builds.

TODO

• ensure we don't watch the fs unless explicitly called by a plugin
• use resolve_specifier and resolve_node_specifier
• maybe add find_unused_modules.task.ts that uses the module graph that we need as a simple usecase