Feature: Mermaid pan & zoom

[snopan]

This PR adds pan & zoom functionality in mermaid MD preview. There's a previous PR that attempted to add this functionality but had some issues. This PR tries address the issues mentioned in the comments.

#125

The changes include:

• Rendering a button about the diagram when there are no errors in diagram
• When svg-pan-zoom hasn't initiated, the behavior should be same as before
• Only when the button is pressed, svg-pan-zoom is initiated allowing for pan and zoom. The sizing of diagram will be set to what it is at that point and it also starts recording any actions taken (pan/zoom) into ZoomStates
• When the markdown is re-rendered, it will go check ZoomStates and sync back to where it is allowing for zoom and editing at the same time
• Unrelated it also removes the throw error when a diagram fails to parse. From what I can see when mermaid library fails to parse the contents, we would catch the error and return a error message content. But for some reason it makes another throw error, so when you have multiple diagrams and one breaks it breaks everything else.

after remove throw error

[snopan]

Hey @mjbvz , when you have some time could you have a look at this PR? Thanks!

[snopan]

Thought I also provide putting the button on the right as option if people prefer this as it's slightly more out of the way

only thing is when diagram is too large, the toggle might move too far right

[snopan]

Recreated PR due to conflicts #295