Fix diff stringification issues and optimized formatting - #602

Resolves #588
Resolves #601

Changes

• Fix duplicate +
• Fix - not stringified.
• Fix +- not at the beginning of the line when stringifying.
• Fix +- escaping
• Fix :: - rendering
• Optimize diff format
• and maybe more?

Known Issues

1. If we directly call the stringify function of the script and block, we will get a string with DIFF_MARK.
   Do we need to handle this situation, or at least export prettyPrintDiff?
2. The - diff of the embedded block still cannot be rendered: say (size :: -)
   (I'm not familiar with rendering, but I might fix it later.)

Details

Fix duplicate +

original code & stringify (fixed):

+ forever
+   show
+ end

stringify (before):

+ forever
+   + show
+ end

Fix - not stringified

original code & stringify (fixed):

- show

show :: -

stringify (before):

show

show

Fix +- not at the beginning of the line when stringifying

original code & stringify (fixed):

  forever
+   show
  end

stringify (before):

forever
  + show
end

Fix +- escaping

original code & stringify (fixed):

\+ show
\- show

stringify (before):

+ show
- show

Fix :: - rendering

code: show :: -
before:

fixed:

Optimize diff format

new (stringify):

  show
+ hide
  hide
+ show

old (stringify):

show
+ hide
hide
+ show

NOTE: The current logic is that if the script has diffs, the entire script is indented. Perhaps it could be changed to be based on doc?

And I think we should add some tests?