'main': Do not look for metacharacters in parameter expansions.

Fixes the bug the previous commit added a test for.

highlighters/main/test-data/parameter-expansion-untokenized1.zsh

@@ -33,6 +33,6 @@ local x="()"
 BUFFER=$'$x ls'
 
 expected_region_highlight=(
-  '1 2 unknown-token "fixed in the next commit"' # $x
+  '1 2 unknown-token' # $x
   '4 5 command' # ls
 )

highlighters/main/test-data/parameter-expansion-untokenized2.zsh

@@ -33,6 +33,6 @@ local x="^foo^bar"
 BUFFER=$'$x ls'
 
 expected_region_highlight=(
-  '1 2 unknown-token "fixed in the next commit"' # $x
+  '1 2 unknown-token' # $x
   '4 5 default' # ls
 )