feat: callouts extension to Markdown (note, tip, warning, etc.)

[jaredcwhite]

Across various editors and displayers (?) of Markdown, there's been a growing consensus around an extension to blockquote syntax that provides for the visual display of a "callout".

Callouts in Bear (as of this year):

Callouts in Obsidian:

I don't know if GitHub invented this syntax or merely popularized it, but at any rate…I came up with a solution in one wiki-style site and it'd be great to move this into core as an optional initializer (similar to #1027).

Some considerations:

• What icons would we use?
• How would we provide default styles?
• Which variations do we support, or is this extensible in some fashion?
• Are there subtle differences between various implementations of this Markdown syntax?