Add json import

[dylan-chong]

Added JSON import/export page.

• the user can paste in some json to set the ranges/board/tree config.
  • this is to allow external tools to generate the json for a quick input
• the json is automatically updated whenever any range/board/tree config is changed. for easy export

requires #31 to be merged first, as this is branched off that
fixes b-inary/wasm-postflop#17

[dylan-chong]

Currently, the added/removed lines is not quite well supported, as it does not validate those. What would you recommend we do about those?

[b-inary]

I am very sorry, but I do not have time to review this pull request and have decided not to merge it. It is well written from what I have skimmed, and should be a much appreciated contribution.

As for the added/removed lines, I improved input validation with commit d640a3b. This should make the program much less likely to crash.

[dylan-chong]

Awesome, have rebased (and fixed a few bugs)

[dylan-chong]

Marking as draft as there is a few type/lint niggles to fix

[dylan-chong]

@b-inary I've finished fixing up the lint issues. No hurry to get this merged, but would appreciate the review if you ever get the time.

If you need me to test any particular scenarios, I'm happy to do that and document the results