Add buttonTagHelper and submitTagHelper for formaction

[ctyar]

Addresses #1668

[dougbu]

Suggest renaming existing resources e.g. AnchorTagHelper_CannotOverrideHref and reusing them here. Resources are internal so there's no compat worry. And maintenance will be easier if we keep the numbers and repetitions down.

[dougbu]

🙈 Never mind. "a" versus "an" and the different ordering of the FormTagHelper_CannotOverrideAction parameters makes this difficult.

[dougbu]

I haven't looked at the tests in detail. But the general direction is very solid.

One question: The <a> and <form> tag helpers already duplicate each other a fair amount. The new <button> and <submit> helpers both look almost identical to the <a> one. Could we refactor things e.g. to a static class? That way, future fixes will be done everywhere and we won't miss a tag helper or three? (Can't change the existing tag helpers to have a base class because that would be a breaking change and @rynowak would hate me forever more.)

[Eilon]

@dougbu assigning this to you since you already started looking at it.

@ctyar - sorry for the delay getting back to you. Do you have thoughts on @dougbu 's comments?

[dougbu]

@ctyar thanks very much for your submission. Please see #5201 for the direction we're taking this work. Your commit remains and will be part of the final push to GitHub of course.

[ctyar]

@dougbu Sorry that couldn't finish the work, I hope this does not cause any inconvenience to you guys