Warn user if .vala file hasn't been added to meson

[Cogitri]

Describe the bug
Right now if the user creates a new .vala file and forgets to add it to meson, code completion doesn't work in the file and every type is interpreted as null. It's easy enough to recognise this and add the missing file afterwards, but I think it'd be nice if VLS issued a warning if the user edits a (new) .vala file that hasn't been added to meson yet. But of course this can be low-priority :)

To Reproduce
Source code repo:

Steps to reproduce the behavior:

1. Create new vala file
2. Start editing, all types show up as null <- Would be nice if VLS issued a warning here
3. Add to meson file
4. It works now

[Prince781]

I think we'd want to have VLS only warn you when you have errors that could be resolved by adding the file to the build system. Otherwise this could be annoying.

Perhaps what we'd want is for VLS to tell from the error messages when a certain namespace is being relied on that exists in one of the other projects that VLS knows about.

[gavr123456789]

wow, really nice idea, never throw about it