Add Lark to code.cs61a.org

[rahularya50]

• [x] Whitelist imports, don't ban them
• [x] .begin, .end for multiline
• [x] Some sort of doctests (%expect?)

Features:

• Server-side Lark interpreter that runs .lark files or autodetected (basically if they contain?start)
• Multi-line input like:
• Imports from common are supported
• Doctests work like this:

  %doctest
  lark> (+ 1 2
  ....> )
  calc_op
  (
  +
  operand   1
  operand   2
  )
  lark> (+ 1 2)
  calc_op
  (
  +
  operand   8
  operand   2
  )
  %end

  put anywhere in a grammar. Multiple doctest blocks can be included. To give them names, write %doctest <NAME>.

To get the expected output in a console, run .toggleviz to disable tree output:

In the doctest pane, clicking the debug icon will display a tree representation of the actual output (or outputs, if you debug an entire test case), and drop you into a lark> console:

[pusherbot]

All PR builds shut down.