Refactor shortcode parsing

[maxime-rainville]

Our approach to managing short code is dated and uses many anti-patterns.

Each individual shortcode class must implement the ShortcodeHandler interface. This interface requires the usage of static method. Nowadays, we prefer to use the singleton pattern and use instance methods.

Short code must be registered in _config.php by making this kind of calls: https://github.com/silverstripe/silverstripe-framework/blob/8886a3a93ddf5d9ad0b677f7403ca600894bbe00/_config.php#L14-L15

Registering shortcodes should be handle via the Injector.

Most of the logic in ShortcodeParser is dedicated to parsing BBCode shortcodes. There's libraries around for this exact purpose:

• thunderer/shortcode
• s9e/text-formatter

We should consider adopting them instead of maintaining our own parser.

[maxime-rainville]

I had a very quick look at the two libraries:

• thunderer/shortcode seems very aligned with our current approach, but it hasn't been updated in two years.
• s9e/text-formatter has been updated in the last month but it seems like quite a departure from the current approach. It seems to prefer a "template" approach that might not be suitable for our uses case.

[GuySartorelli]

Each individual shortcode class must implement the ShortcodeHandler interface.

That's not true - shortcode parsers don't even need to be classes. You only register a callback. It could even be an anonymous function with the current setup.

But yes, I agree we should offload this to a third-party library if one is available and appropriate.

Alternatively we should allow registering them with yaml config, even if we change nothing else about our current approach.