Ensure that all import and require statements, in the entire code-base, have a .js file extension

In order to eventually get rid of SystemJS and start using native `import`s instead, we'll need to provide "complete" file identifiers since otherwise there'll be MIME type errors when attempting to use `import`.

test/unit/unicode_spec.js

@@ -16,14 +16,14 @@
 import {
   getDingbatsGlyphsUnicode,
   getGlyphsUnicode,
-} from "../../src/core/glyphlist";
+} from "../../src/core/glyphlist.js";
 import {
   getNormalizedUnicodes,
   getUnicodeForGlyph,
   getUnicodeRangeFor,
   mapSpecialUnicodeValues,
   reverseIfRtl,
-} from "../../src/core/unicode";
+} from "../../src/core/unicode.js";
 
 describe("unicode", function() {
   describe("mapSpecialUnicodeValues", function() {