highlighter: Fix regions and patterns inside regions (#2840)

* highlighter: Fix regions and patterns inside regions

* highlighting: Remove 2nd recursive highlightRegion() call

...and add limitGroup checks to pattern search.

* yaml: Add TODO type highlighting

* highlighting: Don't stop in highlightRegion() at empty lines

...because possible region line end pattern must be detected.

* syntax/sh: Correct string handling due to additional pattern handling

* syntax/sh: Remove slash in variables

* highlighter: Accept nested region only in case it's within the current reagion

* highlighter: Accept nested patterns only in case it's within the current reagion

* highlighter: Don't search for nesting in case the region end was found at start

runtime/syntax/yaml.yaml

@@ -30,6 +30,5 @@ rules:
     - comment:
         start: "#"
         end: "$"
-        rules: []
-
-
+        rules:
+            - todo: "(TODO|XXX|FIXME):?"