Support \( and fix instant mode switching for $, \(, \text (#1213)

* Fix #1212 by supporting `\(...\)` in addition to `$...$` inline math nested inside `\text`. This turned out to be harder than one would think because of the way `$` was being handled specially, while `\(` needed to be handled as a function, which immediately consumed the `\(` token, which meant that the first following token got consumed in text mode instead of math mode (unlike `$` where we could switch the mode before consumption).
* Added new `consumeMode` setting for functions which tells the parser to switch modes just for the consumption of the command token.
* Now that we're working with functions, move all the `$` handling code into `functions/math.js`.  Somewhat bizarrely, we can define `$` as a function (nearly identical to `\(`) and it all works (just like we can have single-character macros).  This move removed a lot of messy stuff from `Parser.js`: `ParsedDollar`, `ParsedFuncOrArgOrDollar`, `newDollar`, `assertFuncOrArg`, and at least three explicit checks for `if (... === "$")`.
* Moved the `consume()` for the command token from `parseSymbol` to `parseGivenFunction`.  This ended up not being strictly necessary, but seems conceptually cleaner.
* Partially address #1027 by setting `consumeMode: "text"` for `\text` commands.  As a result, `\text` now instantly switches the mode to `"text"`, so even an unbraced macro argument will properly detect text mode.  I added a test for this, which is a slightly weaker form of #1027.

src/types.js

@@ -23,4 +23,5 @@ export type ArgType = "color" | "size" | "url" | "original" | Mode;
 // LaTeX display style.
 export type StyleStr = "text" | "display";
 
-export type BreakToken = "]" | "}" | "$";
+// Allowable token text for "break" arguments in parser
+export type BreakToken = "]" | "}" | "$" | "\\)";