Add sequence & workflow diagram support to Markdown

[cilerler]

Supporting syntax through jsSequence and jsFlow would be great.

related #2273

```sequence
Andrew->China: Says Hello
Note right of China: China thinks\nabout it
China-->Andrew: How are you?
Andrew->>China: I am good thanks!
``

```flow
st=>start: Start:>http://www.google.com[blank]
e=>end:>http://www.google.com
op1=>operation: My Operation
sub1=>subroutine: My Subroutine
cond=>condition: Yes
or No?:>http://www.google.com
io=>inputoutput: catch something...

st->op1->cond
cond(yes)->io->e
cond(no)->sub1(right)->op1
``

[chemaoxian]

yes , it's just what i want

[kaipaysen]

I'd love that feature!

[mjbvz]

Please take a look at our experimental markdown extension exploration in VSCode 1.11: #22916

For example, here's an experimental extension that adds mermaid diagram support to markdown files using this API: https://github.com/mjbvz/vscode-markdown-mermaid

If someone is interested in prototyping an extension for jsflow or jssequence, I'd be happy to help out and hear any feedback you have on the API. The mermaid extension is probably a good staring point

[seanpk]

I am quite interested in having any diagram support in my markdown (I'm not particular about the syntax or underlying technology).
At a basic level, I'd like to be able to replicate what stackedit.io can do for sequence diagrams and flow charts.

@mjbvz - is markdown-mermaid still based on experimental features as indicated in its readme?
What are the next steps for getting a stable extension in place?

Also, related but not necessarily connected to the extension, I wonder what the best practices for doing builds of such documents would look like.

[mjbvz]

@seanpk The markdown extensions API is now enabled by default in the current VS Code insiders builds. I've update the VSCode markdown mermaid extensions to support this. I plan on releasing the extension on the VS Code marketplace after 1.15 ships

Closing this issue since extensions should now be able to contribute this functionality

[seanpk]

thanks @mjbvz, that sounds great