Add building dmd with dub

[dkorpel]

Proof of concept, based on my DConf 23 lightning talk.

I don't want this to be a subpackage, but I presume removing the default 'targetType: none' and frontend dependency will break using dmd as a library by depending on "dmd".

Works on my Windows machine, I still need to test on Posix and add to CI.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15566"

[maxhaton]

Suggested change

	With a pre-built D compiler and dub installed, dmd can be built with:
	With a D compiler and dub installed, dmd can be built with:

[maxhaton]

@Geod24

[maxhaton]

Can it invoke a single step from build.d

[dkorpel]

I don't know, the goal is to move away from build.d