Make meta-builds (`meta-build/build.mill`) configurable

[lefou]

Before https://github.com/com-lihaoyi/mill/pull/4969, a meta-build needed to be enabled explicitly (with import $meta._).

Now, it will be used when found, which seems convenient in most cases. But it also make reasoning about a build harder, since a potential new developer needs to inspect the project to get the full picture. There is also no current way to disable this features.

I propose the add a way to configure it, to make reasoning easier.

Explicitly enable the meta-builds. Not finding one should be an error.

//| meta-build: true

Explicitly disable the meta-build. Mill should not search for and not use it.

//| meta-build: false

Future ideas: Configure the location of the meta-build

//| meta-build: "subpath-to-metabuild"