Liara

* Bump minimum Python version to 3.10.
* Improve log output for shortcodes. Use `--debug` to see information about each shortcode invocation.

* Provide the current node as a context variable to shortcode handlers.

* Rewrite the `ShortcodePreprocessor`. This resolves many issues with the existing preprocessor, for example:

* Quoted strings containing `/%>` are handled correctly now.
* Multiple shortcodes can be used in one line.
* No whitespace is needed before the closing tag.
* Non-quoted values can contain `-` now (previously, it was an alphanumeric characters or `_` only)
* Parsing robustness has generally improved with better error messages.

* Fix the `liara.signals.commandline_prepared` signal not specifying a `sender`, unlike all other signals.
* Improve documentation of signals & plugins.

* Add `exclude` and `include` to thumbnail definitions to skip creation of thumbnails for some images.

* Add support for "shortcodes", that is, function calls embedded in the document source.
* Add a new configuration option to load plugins from a directory.
* Documents which fail to parse don't abort the build any more. Instead, a warning will be printed pointing to the broken document, and the processing will continue.
* Markdown extensions can be configured now. As part of this change, the default output format changed to ``html5`` (previously it was ``xhtml``.)
* Add a new ``build_context`` global variable to templates.