Adapt helpful--keymaps-containing for Emacs 30+

fix: Improve variable setting logic in helpful.el

handle package-version safely to prevent listp error

Fix scan-sexps error for C-defined variables

Fix wrong-type-argument error in helpful-key

Use setopt for "set" button if available

Test CI on Emacs 29

Repair helpful--display-implementations test for emacs 29.1

Look for "function" or "variable" text in doc comments case-insensiti…

Remove broken tokei LOC badge

fix nil pos when building button for vars

Enable outline-support for Emacs 29

Mark customizable variables (user options) in the summary line

Support describing faces using helpful-symbol.

Roll version for 0.22

Update changelog for 0.21

0.20 has been released, roll version

helpful--heading: propertize newline as well, enabling :extend t

suppress some compiler warnings

Remap revert-buffer and revert-buffer-quick to helpful-update.