Markdown Support for Chatroom and Stylistic Changes

[AnoldGH]

Added markdown support to chatroom with solid markdown and remarkGfm.
Chatroom can now display Markdown as well as its Github extensions:

Also added are some stylistic changes inspired by Github UI. Messages from local user are displayed in a light blue box much like Github discussions, while messages from foreign users are displayed in black and white.

[zjkmxy]

Thanks, but you moved too fast. Let's first discuss if we should go this direction.

This is a temporary hold. No actual change requested.

[AnoldGH]

Sure thing. This PR mainly touches on two parts:

1. Adds Markdown support. This was a TODO label in the original chat.tsx, which I think could be useful. This code here does it:
   

   ndn-workspace-solid/src/components/chat/chat.tsx

   Lines 108 to 112 in a1fe54d

   	<SolidMarkdown
   	children={msg.value.content}
   	remarkPlugins={[remarkGfm]}
   	components={{ code: Code }}
   	/>

   
   solid-markdown claims they are 90% similar to the original react-markdown. During my tests I didn't encounter any notable issues, and so I decided to go with it.

2. Stylistic changes
   I decided to make these changes upon realizing Markdown display doesn't go well with the original style, since the message box was wrapped in different colors and it was hard to distinguish between codes and texts.
   Several assumptions I made during the changes:

   1. Workspace is used mainly during collaborative working, so the message box should be clear to display full segments of code, latex formula, etc.
   2. Since it is more often a working environment, some inconvenience for casual talks can be tolerated.
   3. The lack of profile choosing features is likely to continue, so the sender's username must be displayed somewhere clear

   I find Github UI and some online forums to be rather good references for these requirements, and made the changes inspired by these designs.

[zjkmxy]

I don't recommend to create your own color instead of using the one managed by global themes. However, since the existing chat already created a lot of colors (very bad experience in dark theme), I will accept this.

[AnoldGH]

It indeed looks atrocious in dark theme, I will see what I can do later