Unicode characters in math render in text mode (#2040)

* Unicode characters in math render in text mode

Improve #2031 by rendering all supported Unicode text characters (via
supportedCodepoints) in text mode, mimicking wrapping them in `\text`,
thereby using metrics of letter M as usual.

* Add tests

* Improve documentation

* Implement review comments

src/Parser.js

@@ -971,9 +971,16 @@ export default class Parser {
                         nucleus);
                 }
             }
+            // All nonmathematical Unicode characters are rendered as if they
+            // are in text mode (wrapped in \text) because that's what it
+            // takes to render them in LaTeX.  Setting `mode: this.mode` is
+            // another natural choice (the user requested math mode), but
+            // this makes it more difficult for getCharacterMetrics() to
+            // distinguish Unicode characters without metrics and those for
+            // which we want to simulate the letter M.
             symbol = {
                 type: "textord",
-                mode: this.mode,
+                mode: "text",
                 loc: SourceLocation.range(nucleus),
                 text,
             };

src/buildCommon.js

@@ -80,9 +80,8 @@ const makeSymbol = function(
             metrics.width, classes);
     } else {
         // TODO(emily): Figure out a good way to only print this in development
-        typeof console !== "undefined" && console.warn(
-            "No character metrics for '" + value + "' in style '" +
-                fontName + "'");
+        typeof console !== "undefined" && console.warn("No character metrics " +
+            `for '${value}' in style '${fontName}' and mode '${mode}'`);
         symbolNode = new SymbolNode(value, 0, 0, 0, 0, 0, classes);
     }