Some more prep for revamped sourcegens and CI

[dodexahedron]

This issue is primarily for the next piece of preparation for a couple of other issues, such as splitting off the enum generators and enhancements to the CI pipeline that kinda come along for free with that.

But it's also a good spot to bring in a couple other related but small pieces of work which also share the goal of simplification of the build and also the consumer's experience with the packaged library.

At a high level, this one will come with at least:

• Additional configurations added to the solution and projects for which they are relevant.
  • At minimum, some specifically meant for builds/tests/deployments in CI, which also aim to improve the design-time experience by making that stuff not part of local builds unless you want it to be, by choosing the CI configurations.
• Take a pass over dependencies and reduce where feasible.
  • Highest priority there being reducing run-time dependencies on code consuming TG (such as the root of v2 crashes on run with missing dependency JetBrains.Annotations #3544).
• Include a pre-packaged form of the enum generators and remove the projects from the solution.
  • Also going to drop a README.md in the Analyzers and project directories under Analyzers with a note that they're deprecated and slated for replacement soon, just in case someone comes along and wants to add onto them or something. 🤷‍♂️
• Declare existing classes in the Roslyn projects as partial and add a conditionally-compiled file that adds [Obsolete] to all of them, with the condition not being true by default until later, by flipping a switch, when the change to using the external package is almost ready to go.
  • I'll make a PR for that switch flip when it's time.
  • This is really just an insurance policy, but it's almost 0-effort, so might as well go for it.
• Probably other minor stuff I'll mention if/when they come up.

[dodexahedron]

A few of the mentioned items are already done in my working tree, but I've got plenty more to do for that task list above.

[dodexahedron]

Oh hey.

Sorry I was busy and then out of town and then busy again.

But update here.

For sake of simplicity I figured I'd just remove them altogether for now and they can be referenced again closer to release or even in a point release, since it's purely optimization anyway. Yes, sometimes significant, but still not a huge deal. 🤷‍♂️

Any objections?