Report sub-token error locations again

This fixes a regression about where we report errors:

	echo error(here
	old: ^
	   fixed: ^

Commit 0c22f67bd (Remove the old parser bits, 2020-07-02) removed
uses of "error_offset_within_token" so we always report errors at
token start. Add it back, hopefully restoring the 3.1.2 behavior.

Note that for cases like

	echo "$("

we report "unbalanced quotes" because we treat the $( as double
quote.  Giving a better error seems hard because of the ambguity -
we don't know if quote is meant to be inside or outside the command
substitution.

tests/checks/syntax-error-location.fish

@@ -22,3 +22,13 @@ echo '
 # CHECK: <fish: Command substitutions not allowed>
 # CHECK: <(true one)>
 # CHECK: <^>
+
+$fish -c 'echo "unfinished "(subshell' 2>| string replace -r '.*' '<$0>'
+# CHECK: <fish: Unexpected end of string, expecting ')'>
+# CHECK: <echo "unfinished "(subshell>
+# CHECK: <                  ^>
+
+$fish -c 'echo "unfinished "$(subshell' 2>| string replace -r '.*' '<$0>'
+# CHECK: <fish: Unexpected end of string, expecting ')'>
+# CHECK: <echo "unfinished "$(subshell>
+# CHECK: <                   ^>