Create Whitespace grammar productions

I created productions for `END_OF_LINE`, `IGNORABLE_CODE_POINT`, and
`HORIZONTAL_WHITESPACE` as that is how the unicode standard is written
and in preparation for #1974 which will make use of
`HORIZONTAL_WHITESPACE`

Author: epage

src/notation.md

@@ -57,10 +57,6 @@ r[input.syntax]
 NUL -> U+0000
 
 TAB -> U+0009
-
-LF -> U+000A
-
-CR -> U+000D
 ```
 
 [binary operators]: expressions/operator-expr.md#arithmetic-and-logical-binary-operators

src/whitespace.md

@@ -1,21 +1,60 @@
 r[lex.whitespace]
 # Whitespace
 
+r[whitespace.syntax]
+```grammar,lexer
+@root WHITESPACE ->
+      END_OF_LINE
+    | IGNORABLE_CODE_POINT
+    | HORIZONTAL_WHITESPACE
+
+LF -> LINE_FEED
+
+CR -> CARRIAGE_RETURN
+
+END_OF_LINE ->
+      LINE_FEED
+    | VERTICAL_TAB
+    | FORM_FEED
+    | CARRIAGE_RETURN
+    | NEXT_LINE
+    | LINE_SEPARATOR
+    | PARAGRAPH_SEPARATOR
+
+LINE_FEED -> U+000A
+
+VERTICAL_TAB -> U+000B
+
+FORM_FEED -> U+000C
+
+CARRIAGE_RETURN -> U+000D
+
+NEXT_LINE -> U+0085
+
+LINE_SEPARATOR -> U+2028
+
+PARAGRAPH_SEPARATOR -> U+2029
+
+IGNORABLE_CODE_POINT ->
+      LEFT_TO_RIGHT_MARK
+    | RIGHT_TO_LEFT_MARK
+
+LEFT_TO_RIGHT_MARK -> U+200E
+
+RIGHT_TO_LEFT_MARK -> U+200F
+
+HORIZONTAL_WHITESPACE ->
+      HORIZONTAL_TAB
+    | SPACE
+
+HORIZONTAL_TAB -> U+0009
+
+SPACE -> U+0020
+```
+
 r[lex.whitespace.intro]
 Whitespace is any non-empty string containing only characters that have the
-[`Pattern_White_Space`] Unicode property, namely:
-
-- `U+0009` (horizontal tab, `'\t'`)
-- `U+000A` (line feed, `'\n'`)
-- `U+000B` (vertical tab)
-- `U+000C` (form feed)
-- `U+000D` (carriage return, `'\r'`)
-- `U+0020` (space, `' '`)
-- `U+0085` (next line)
-- `U+200E` (left-to-right mark)
-- `U+200F` (right-to-left mark)
-- `U+2028` (line separator)
-- `U+2029` (paragraph separator)
+[`Pattern_White_Space`] Unicode property.
 
 r[lex.whitespace.token-sep]
 Rust is a "free-form" language, meaning that all forms of whitespace serve only