bug(schematics): themingApi missing from schematics

Ever since this commit, we can no longer run the themingApi schematic outside of ng update. This means that if you have a codebase with just scss, you will no longer be able to convert to ng12. One use-case is that you have a theme in its own repository or module, and import it in the projects where you need it.

So my proposal is: bring it back. I don't see any reason to remove it as a standalone schematic, given its usefulness.

If bringing it back is a problem, then it should at least be removed from the documentation. I spent several hours trying to figure out where this darn themingApi is at.

I think it makes more sense to just read the above description, but here's the template stuff:

Reproduction

Steps to reproduce:

1. make a project with just @angular/material^12
2. run ng g @angular/material:themingApi

Expected Behavior

Angular CLI converts the scss to new style, as advertised here.

Actual Behavior

Error:

An unhandled exception occurred: Schematic "themingApi" not found in collection "@angular/material".

Environment

• Angular: n/a
• CDK/Material: 12.0.0-rc3 and above
• Browser(s): n/a
• Operating System (e.g. Windows, macOS, Ubuntu): Windows 10

Workaround for now, for anyone visiting: use version 12.0.0-rc2 and run ng g @angular/material:themingApi - it should work, although it might be buggy compared to the current script.

Alternatively, copy the scss files into a valid angular project and run ng update --from=11 @angular/material@12 --migrate-only.

[AndyClausen]

Leaving a comment here to let you know that this is still a thing and that I'm the author of this issue - merged work and personal github accounts together, and lost this issue in the process.