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.

test/katex-spec.js

@@ -812,7 +812,6 @@ describe("A text parser", function() {
     const badTextExpression = "\\text{a b%}";
     const badFunctionExpression = "\\text{\\sqrt{x}}";
     const mathTokenAfterText = "\\text{sin}^2";
-    const textWithEmbeddedMath = "\\text{graph: $y = mx + b$}";
 
     it("should not fail", function() {
         expect(textExpression).toParse();
@@ -872,7 +871,40 @@ describe("A text parser", function() {
     });
 
     it("should parse math within text group", function() {
-        expect(textWithEmbeddedMath).toParse();
+        expect("\\text{graph: $y = mx + b$}").toParse();
+        expect("\\text{graph: \\(y = mx + b\\)}").toParse();
+    });
+
+    it("should parse math within text within math within text", function() {
+        expect("\\text{hello $x + \\text{world $y$} + z$}").toParse();
+        expect("\\text{hello \\(x + \\text{world $y$} + z\\)}").toParse();
+        expect("\\text{hello $x + \\text{world \\(y\\)} + z$}").toParse();
+        expect("\\text{hello \\(x + \\text{world \\(y\\)} + z\\)}").toParse();
+    });
+
+    it("should forbid \\( within math mode", function() {
+        expect("\\(").toNotParse();
+        expect("\\text{$\\(x\\)$}").toNotParse();
+    });
+
+    it("should forbid $ within math mode", function() {
+        expect("$x$").toNotParse();
+        expect("\\text{\\($x$\\)}").toNotParse();
+    });
+
+    it("should detect unbalanced \\)", function() {
+        expect("\\)").toNotParse();
+        expect("\\text{\\)}").toNotParse();
+    });
+
+    it("should detect unbalanced $", function() {
+        expect("$").toNotParse();
+        expect("\\text{$}").toNotParse();
+    });
+
+    it("should not mix $ and \\(..\\)", function() {
+        expect("\\text{$x\\)}").toNotParse();
+        expect("\\text{\\(x$}").toNotParse();
     });
 
     it("should parse spacing functions", function() {
@@ -2824,8 +2856,13 @@ describe("A macro expander", function() {
             {"\\mode": "\\TextOrMath{text}{math}"});
     });
 
-    // TODO(edemaine): This doesn't work yet.  Parses like `\text math`,
-    // which doesn't even treat all four letters as an argument.
+    it("\\TextOrMath should work in a macro passed to \\text", function() {
+        compareParseTree("\\text\\mode", "\\text t",
+            {"\\mode": "\\TextOrMath{t}{m}"});
+    });
+
+    // TODO(edemaine): This doesn't work yet.  Parses like `\text text`,
+    // which doesn't treat all four letters as an argument.
     //it("\\TextOrMath should work in a macro passed to \\text", function() {
     //    compareParseTree("\\text\\mode", "\\text{text}",
     //        {"\\mode": "\\TextOrMath{text}{math}"});