check_spelling: various improvements

- Use the custom dictionary when checking hyphenated words.
- Add the custom dictionary to the existing dictionary so
  suggestions will include words from the custom dictionary.
- Refactor spell checking functions into wrapper calls.

Removes over 200 false positives.

check_source/check_spelling_c_config.py

@@ -111,7 +111,7 @@ dict_custom = {
     "parameterization",
     "parentless",
     "passepartout",
-    "pixelated", "pixelisation",
+    "pixelate", "pixelated", "pixelisation",
     "planarity",
     "polytope",
     "postprocessed",
@@ -422,3 +422,14 @@ dict_ignore = {
     "bugprone-suspicious-enum-usage",
     "bugprone-use-after-move",
 }
+
+# Allow: `un-word`, `re-word` ... etc, in this case only check `word`.
+dict_ignore_hyphenated_prefix = {
+    "de",
+    "mis",
+    "non",
+    "post",
+    "pre",
+    "re",
+    "un",
+}