badwords: check indented lines in source code, fix fallouts

- badwords.pl: add `-a` option to check all lines in source code files.
  Before this patch indented lines were skipped (to avoid Markdown code
  fences.)
- GHA/checksrc: use `-a` when verifying the source code.
- GHA/checksrc: disable `So` and `But` rules for source code.
- GHA/checksrc: add docs/examples to the verified sources.
- badwords.txt: delete 4 duplicates.
- badwords.txt: group and sort contractions.
- badwords.txt: allow ` url = `, `DIR`, `<file name`.

Closes #19536

docs/examples/htmltidy.c

@@ -50,7 +50,7 @@ void dumpNode(TidyDoc doc, TidyNode tnod, int indent)
   for(child = tidyGetChild(tnod); child; child = tidyGetNext(child) ) {
     ctmbstr name = tidyNodeGetName(child);
     if(name) {
-      /* if it has a name, then it's an HTML tag ... */
+      /* if it has a name, then it is an HTML tag ... */
       TidyAttr attr;
       printf("%*.*s%s ", indent, indent, "<", name);
       /* walk the attribute list */
@@ -62,7 +62,7 @@ void dumpNode(TidyDoc doc, TidyNode tnod, int indent)
       printf(">\n");
     }
     else {
-      /* if it does not have a name, then it's probably text, cdata, etc... */
+      /* if it does not have a name, then it is probably text, cdata, etc... */
       TidyBuffer buf;
       tidyBufInit(&buf);
       tidyNodeGetText(doc, child, &buf);