feat: add abnf and railroad diagram

[DecimalTurn]

First version of the ABNF for JSONC.

Note that the current vesion:

• Doesn't support trailing commas. That's because this will be the ABNF for v0. Trailing commas will be part of v1.

• Includes the notion of Line Seperator (LS) and Paragraph Seperator (PS) since that is part of the EcmaScript standard. The current approach is to accept them as comment terminator. Note that if they are not considered as comment terminator and can live inside single-line comemnts, someone could craft a malicious JSONC comment that would include those characters and could hide JS code that would be executed if used inside a JavaScript file.

  If LS is not a comment terminator, but is allowed to appear on a single-line comment line as part of the comment, you could have the following:

  // test.jsonc
  {
    // U+2028 ->
 console.log('rce');
  }

  Which would be valid JSONC with a simple harmless comment inside an empty object. However, if transferred to a JS file or run form Node,js, it would cause the command to execute (ie. node test.jsonc would output this:

  rce
  

[DecimalTurn]

PR preview isn't working well, so I've merged and the railroad diagram is now visible here:
https://jsonc.org/grammar/railroad-diagram.html

[DecimalTurn]

Note that this was merged to facilitate the viewing experience for the railroad diagrams, but the abnf might still need some work before release.