Split up WHITESPACE

I'm splitting out `HORIZONTAL_WHITESPACE` to make it easier to connect
the concept in frontmatter with `WHITESPACE`.

In doing this, I found it awkward to only pull out part when the
comments are there.
I either needed a redundant comment to start a section, re-arrange out
of order from Unicode, or split out all classes.
I did the latter.

Author: epage

src/whitespace.md

@@ -4,19 +4,25 @@ r[lex.whitespace]
 r[whitespace.syntax]
 ```grammar,lexer
 @root WHITESPACE ->
-    // end of line
+      END_OF_LINE
+    | IGNORABLE_CODE_POINT
+    | HORIZONTAL_WHITESPACE
+
+END_OF_LINE ->
       LF
     | U+000B // vertical tabulation
     | U+000C // form feed
     | CR
     | U+0085 // Unicode next line
     | U+2028 // Unicode LINE SEPARATOR
     | U+2029 // Unicode PARAGRAPH SEPARATOR
-    // Ignorable Code Point
-    | U+200E // Unicode LEFT-TO-RIGHT MARK
+
+IGNORABLE_CODE_POINT ->
+      U+200E // Unicode LEFT-TO-RIGHT MARK
     | U+200F // Unicode RIGHT-TO-LEFT MARK
-    // horizontal whitespace
-    | TAB
+
+HORIZONTAL_WHITESPACE ->
+      TAB
     | U+0020  // space ' '
 
 TAB -> U+0009  // horizontal tab ('\t')