Allow configuring watch-ignored directories in Mill frontmatter

[arturaz]

We ignore out/ in the filesystem watcher by default, as it consumes resources and there is no point in watching that, but a project might contain more directories like that.

For example, my full stack project has:

appClient/vite/node_modules # very large...
appClient/vite/scala_output # generated by scala JS

I suspect there is quite an overhead in watching those, so we could have:

//| mill-watch-ignore:
//| - appClient/vite/node_modules
//| - appClient/vite/scala_output

[lefou]

To my understanding, Mill isn't watching the full project dir by default, but only explicitly configured paths (via source tasks or explicit watch API).