Markdown support

[nickvergessen]

• Overview ticket: Basic text formatting to chat spreed#1027
• API discussion: Markdown support - API spreed#9950

We plan to role out the following support for markdown:

non-optional

• bold
• italic

optional

Optional means it is just shown with the markdown syntax

• strike-through
• list
• todo list with checkboxes
• code block using 3 `
• inline code using `
• quote
• headlines
• dividers ---

[SystemKeeper]

Some research on this.

Apple native markdown implementation:

• bold
• italic
• strike-through
• list
• todo list with checkboxes
• code block using 3 ` (partially, is parsed as markdown, but removes newlines inside the 3 ` which renders it useless)
• inline code using `
• quote
• headlines
• dividers ---

👍 Native
👎 Hard to customize
👎 no newlines inside of the 3 ` renders it useless in my opinion

Down library https://github.com/johnxnguyen/Down

• bold
• italic
• strike-through (see [Commonmark] Strikethrough not working / not supported johnxnguyen/Down#253)
• list
• todo list with checkboxes
• code block using 3 `
• inline code using `
• quote (looks like it's parsed, but has no different styling, could probably be adjusted)
• headlines
• dividers --- (dividers are not visible, but might also be a styling option)

CDMarkdownKit https://github.com/chrisdhaan/CDMarkdownKit

• bold
• italic
• strike-through
• list (no sublists)
• todo list with checkboxes
• code block using 3 `
• inline code using `
• quote
• headlines
• dividers --- (Could possibly be added through custom syntax addition)

👍 Very easy to customize appearance
👎 Need to research missing indentation, spaces at the beginning of a line are removed

Comment

• All 3 ways can parse a NSString to a NSAttributedString, so switching between the 3 ways is easy
• Set up a test with all 3 ways, was very easy in all cases
• CDMarkdownKit is very easily customizable

[SystemKeeper]

👎 Need to research missing indentation, spaces at the beginning of a line are removed

Works by removing
https://github.com/chrisdhaan/CDMarkdownKit/blob/b08981016f9406eb6d18150a3c8caa4288983d2e/Source/CDMarkdownParser.swift#L191-L199

So we could change this locally and provide a patch upstream.

[SystemKeeper]

Done in #1296

[nickvergessen]

There is a markdown property on messages now. If that is false, all Markdown rendering should be disabled:
nextcloud/spreed#10067