Refactor parts of initial KaTeX handling

[gnprice]

This follows up on #1408 (KaTeX part 1) with some refactors that I felt were easiest to express by demonstration. I think it'd be good for the next PR in the series, #1452, to be rebased atop these.

This doesn't include the changes I've sketched for passing the font size down directly through the recursion in the parser and widgets. I'll want to look a bit more at how those would interact with the changes in #1452 (which introduce a number of additional length-valued properties) before I'm confident just what shape that part of the API should take.

Selected commit messages

3ac0037 katex [nfc]: Increment class index at top of loop

This way there's nothing that needs to happen at the bottom of the
loop, if any of the cases matched.

b003242 katex [nfc]: Join the CSS-class switch statements into one

Conveniently, the two redundant rules say the exact same thing
when they apply. So the first one has no effect, and we can
ignore it.

b8186a7 katex [nfc]: Replace classFound local with a default case

Before this change, all cases of this switch statement either
continue the loop, or throw, or set classFound to true.
The error therefore gets logged just if none of the cases matched.

So we can express the same behavior with a default case.

9854bb8 katex [nfc]: Increment class index immediately on dereference

This makes the reasoning about these index values more local.

31e0bf0 katex [nfc]: Handle error immediately on spanClasses overrun

Like an early return, this (a) brings the consequence of the error
immediately next to the condition defining it, and (b) lets the normal
happy case continue vertically down without adding indentation.

d76120a katex [nfc]: Make KatexSpanStyles immutable

Once the parsing is done, we want these to remain unchanged,
just like the other objects in the parse tree.

So, like ContentNode and its subclasses, make the class immutable.
The parser needs to mutate its own draft of what styles to apply
to a given span; but it can do that with its own local variables
corresponding to the fields, and construct a styles object at the
end of the loop.

6bb726a katex [nfc]: Factor out _KatexNodeList widget

This deduplicates the logic for the particular way that a list of
KaTeX nodes get combined into a single widget.

0eb1410 katex [nfc]: Rename _KatexSpan field to "node"

This makes it more uniform with our other content widgets.

[rajveermalviya]

Thanks for working on this @gnprice! Added one comment about another possible cleanup in the tests, otherwise LGTM.

[rajveermalviya]

Nice, now that this is immutable, we can also change the ContentExample tests back to const in test/model/content_test.dart:

-  static final mathBlock = ContentExample(
+  static const mathBlock = ContentExample(

I'll add a commit that does that in the next PR.

[rajveermalviya]

Sent that as d3b21fd in #1452.

[gnprice]

Thanks for the review! Merging.