Markdown toolbar for easy formatting

Could be a useful feature to have a toolbar with buttons that insert Markdown snippets into the textarea you're writing in. Stuff like details/summary in particular. Should probably look similar to GitLab's, although instead of icons I wanna do it with a few commonly-used options in the toolbar as text and a "More" dropdown so we adhere to Tildes' "Use words, not icons" design goal.

The dollar sign shows where the cursor should be placed after inserting the text:

• Link: [$]() (if text was selected, it will be [<text>]($))
• Code: ```\n$\n```
• Strikethrough: ~~$~~
• Spoilerbox: <details>\n<summary>Click to expand spoiler.</summary>\n\n$\n</details>
• Bold: **$**
• Horizontal divider: \n\n---\n\n$
• Inline Code: `$`
• Italic: *$*
• Ordered List: 1. $
• Small: <small>$</small>
• Unordered List: * $