Mark the entire error location with a squiggle

This makes it so instead of marking the error location with a simple
`^`, we mark it with a caret, then a run of `~`, and then an ending `^`.

This makes it easier to see where exactly an error occured, e.g. which
command substitution was meant.

Note: Because this uses error locations that haven't been exposed like
that, it's likely to shake out weirdnesses and inaccuracies. For that
reason I've not adjusted the tests yet.

src/parse_tree.cpp

@@ -126,6 +126,19 @@ wcstring parse_error_t::describe_with_prefix(const wcstring &src, const wcstring
     result.push_back(L'\n');
     result.append(caret_space_line);
     result.push_back(L'^');
+    if (source_length > 1) {
+        // Add a squiggle under the error location.
+        // We do it like this
+        //               ^~~^
+        // With a "^" under the start and end, and squiggles in-between.
+        auto width = fish_wcswidth(src.c_str() + source_start, source_length);
+        if (width > 2) {
+            // Subtract one for each of the carets - this is important in case
+            // the starting char has a width of > 1.
+            result.append(width - 2, L'~');
+            result.push_back(L'^');
+        }
+    }
     return result;
 }