Fix append element to #mock but don't rm it #test

https://github.com/mathquill/mathquill/commit/8fd3b2de#commitcomment-6495760
Fix .cmd() ReferenceError; rm Cursor::insertCmd

Cursor::insertCmd is used by LatexCommandInput::renderCommand and by
EditableField::cmd (the public API). MathQuill-basic doesn't have
TextBlock's, so passing .cmd() an unrecognized LaTeX command actually
causes a ReferenceError right now, but that is the only place that still
references TextBlock, and only in that edge case, so we still def don't
want TextBlock in MathQuill-basic.

So what I do is say, those uses of Cursor::insertCmd should never have
been unified, inline it as-is into LatexCommandInput::renderCommand and
inline the non-TextBlock part into EditableField::cmd (instead, when
passed an unrecognized backslash command, noop, which is at least
consistent-ish with .latex() (which empties the MathQuill if unable to
parse the LaTeX), though proper error reporting would be nice).

I actually really don't like the unknown-backslash-commands-are-turned-
into-TextBlock's behavior (like \asdf turns into \text{asdf}, which I'm
leaving in LatexCommandInput::renderCommand), that's weird and I want to
get rid of it eventually, but what I want to do instead is force the
cursor to stay in the LatexCommandInput and I haven't really thought
through how to do that and it shouldn't matter for now.

Also, unit test all the new behavior, normal and error conditions; there
were basically no unit tests for the old behavior.
Fix LiveFraction referencing TextBlock in MQ-basic

...and throwing a ReferenceError just for typing '/'.

Instead, `instanceof TextBlock` should just silently fail if there's no
TextBlock, because then a node couldn't possibly be an instance of it.

(I was amazed to learn that `instanceof undefined` errors, I totally
thought it fails silently, apparently instanceof anything that's not a
function errors, so just `|| noop`.)

Also added a MathQuill-basic section to the LiveFraction test.

Recommend -w to view diff.
Move {Math,Text}Field out of publicapi.js

...to {math,text}.js, respectively.

This way, rather than MathQuill-basic having an undocumented
MathQuill.TextField that ReferenceError's when called (since
RootTextBlock isn't defined), it doesn't have MathQuill.TextField
at all.

There is now no reference to TextBlock or RootTextBlock whatsoever in
MathQuill-basic.