feat: enhanced header rendering in the chat window

[folke]

As discussed, the separator is now repeated in a virtual line and has hl group CopilotChatSeparator.

The headers have hl_group CopilotChatHeader.

I didn't change the default header strings, so they still have ##, but that's no longer needed to get the same styling.  

With custom header strings:

Closes #334

[GitMurf]

@deathbeam will this effect saving a chat conversation? Historically you could save the buffer as markdown file and have a nicely formatted conversation in markdown. Is that no longer the case if we are removing the ## Heading Syntax?

[deathbeam]

@deathbeam will this effect saving a chat conversation? Historically you could save the buffer as markdown file and have a nicely formatted conversation in markdown. Is that no longer the case if we are removing the ## Heading Syntax?

The syntax is still there this just added some extmarks on top for better visuals and thats it, do not affects how the data are stored at all

[GitMurf]

The syntax is still there this just added some extmarks on top for better visuals and thats it, do not affects how the data are stored at all

Oh ok. I guess I got a little confused with folke's custom header strings screenshot above.