Add support for description lists (<dl>, <dt> and <dd>)

[Steffo99]

Description

This PR adds support for <dl>, <dt>, and <dd> HTML tags in messages.

As with the tables PR, this PR does not add support for editing the description list.

Warning

The Matrix spec seems to recommend not supporting description lists: https://spec.matrix.org/v1.19/client-server-api/#mroommessage-msgtypes

I don't see any risk or harm in implementing them anyway, as they have no way of being a vector for cyberattacks. (Should a MSC be submitted to add them?)

Fixes #2050.

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

AI disclosure:

• Created without the usage of AI
• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).