diagnostics: beware of zero-width characters

Currenly we rely on (visual) width of the characters to decide where
to open and close the styling of the quoted lines.  This breaks when
we deal with zero-width characters: we cannot just rely on (visual)
columns, we need to know whether we are before, inside, or after the
highlighted portion.

* src/location.c (location_caret): col_end: no longer add 1, "regular"
characters have a width of 1, only 0-width characters have 0-width.
opened: replace with 'state', a three-valued enum.
Don't reopen the style if we already did.
* tests/diagnostics.at (Zero-width characters): New.

tests/diagnostics.at

@@ -37,15 +37,15 @@ AT_BISON_OPTION_PUSHDEFS
 
 AT_DATA_GRAMMAR([[input.y]], [$2])
 
+AT_DATA([experr], [$4])
+
 # For some reason, literal ^M in the input are removed and don't end
 # in `input.y`.  So use the two-character ^M represent it, and let
 # Perl insert real CR characters.
-if grep '\^M' input.y >/dev/null; then
-  AT_PERL_REQUIRE([-pi -e 's{\^M}{\r}gx' input.y])
+if $EGREP ['\^M|\\[0-9][0-9][0-9]'] input.y experr >/dev/null; then
+  AT_PERL_REQUIRE([-pi -e 's{\^M}{\r}g;s{\\(\d{3}|.)}{$v = $[]1; $v =~ /\A\d+\z/ ? chr($v) : $v}ge' input.y experr])
 fi
 
-AT_DATA([experr], [$4])
-
 AT_CHECK([LC_ALL="$locale" $5 bison -fcaret --color=debug -Wall input.y], [$3], [], [experr])
 
 # When no style, same messages, but without style.
@@ -193,6 +193,24 @@ input.y:12.8-10:     previous declaration
 ]])
 
 
+## ----------------------- ##
+## Zero-width characters.  ##
+## ----------------------- ##
+
+# We used to open twice the styling for characters that have a
+# zero-width on display (e.g., \005).
+
+AT_TEST([[Zero-width characters]],
+[[%%
+exp: an\005error.
+]],
+[1],
+[[input.y:10.8: <error>error:</error> invalid character: '\\005'
+   10 | exp: an<error>\005</error>error.
+      |        <error>^</error>
+]])
+
+
 ## -------------------------------------- ##
 ## Tabulations and multibyte characters.  ##
 ## -------------------------------------- ##