improvement: markdown shortcuts

[mscolnick]

Adds a few markdown shortcuts

Bold (Mod-b)
Italic (Mod-i)
Convert to Link (Mod-k)
Convert to Ordered list (Mod-Shift-7)
Convert to Unordered list (Mod-Shift-8)
Convert to Blockquote (Mod-Shift-9)
Convert to Code (Mod-Shift-0)

Also improves pasting with a URL when selected and wrapping with a tick (`)

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
marimo-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 17, 2024 2:00pm
marimo-storybook	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 17, 2024 2:00pm

[akshayka]

This feels great!

Found a couple bugs though.

1. I'm unable to copy paste URLs into the markdown editor (nothing happens). Pasting non urls still works.

2. I really like the "to code" shortcut. For fenced blocks (triple ticks), can we add a newline at the beginning and end of the block?

For example, starting from

def foo():
  ...

Instead of

```def foo():
...```

the shortcut should yield

```
def foo():
...
```

[github-actions]

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.6.1-dev1