fix: column_heading does not allow space after "~"

Example from ":help definitions":

    2. bbb		2. bbb		2. bbb		10. ddd
    3. ccc ccc ccc	3. ccc ccc ccc	3. ccc ccc c	11. ~

Author: justinmk

corpus/codespan.txt

@@ -40,11 +40,9 @@ an error`.
         (word))
       (word)
       (word)
-      (codespan
+      (ERROR
         (word)
-        (MISSING "`")))
-    (line
-      (word)
+        (word))
       (word))))
 
 ================================================================================

corpus/heading3-column_heading.txt

@@ -234,6 +234,7 @@ ABC not-h3
 NOT column_heading
 ================================================================================
 tilde ~/foo/bar foo~ bar
+trailing-space-is-NOT-column_heading ~ 
 
 
 --------------------------------------------------------------------------------
@@ -243,5 +244,8 @@ tilde ~/foo/bar foo~ bar
     (line
       (word)
       (word)
+      (word)
+      (word))
+    (line
       (word)
       (word))))

corpus/optionlink.txt

@@ -99,9 +99,13 @@ x `after_codespan`'s
   (ERROR)
   (block
     (line
-      (codespan
+      (argument
         (word))
-      (ERROR))))
+      (word))
+    (ERROR
+      (word)
+      (codespan
+        (word)))))
 
 ================================================================================
 NOT optionlink 3 (FIXME)
@@ -116,6 +120,8 @@ foo '"\ '.  Notice
   (block
     (line
       (word)
-      (word
-        (ERROR)
-        (MISSING "'")))))
+      (ERROR)
+      (word))
+    (line
+      (taglink
+        (word)))))

grammar.js

@@ -137,10 +137,7 @@ module.exports = grammar({
     // TODO: children should be $.word (plaintext), not $.atom.
     column_heading: ($) => seq(
       field('name', seq(choice($._atom_noli, $._uppercase_words), repeat($._atom))),
-      choice(
-        token.immediate(/~[\t ]*\n/),
-        /~[\t ]*\n/,
-      ),
+      /~\n/,
     ),
 
     h1: ($) =>