Allow using filenames in codeblocks

[WebFreak001]

Description

Makes it possible to send code blocks as ```filename.json or ```src/app/example.js and having Cinny display the filename in the header of code blocks as well as only using the text after the dot as the language specifier.

• If there is a dot in the syntax name, we instead treat it as the filename and only everything after the last dot as the syntax name
• we use a custom "data-label" attribute on the code block to specify the name of the file (so only compatible with new Cinny versions, no support for other matrix clients and no spec/standard for this)

Preview in Cinny:

 

Preview in old Cinny and probably other clients:

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

Preview: https://2455--pr-cinny.netlify.app
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[ajbura]

While this can work within cinny this will break the every other client code highlight

[ajbura]

While this looks fine it doesn't include changes to retrieve filename when editing a message.