Implement ASI in js_parser/parser.py

[jorendorff]

There are so many bits and pieces here:

• [x] before offending token after LineTerminator
• [x] before }
• [x] at end of do-while statement
• [x] at end of source text
• [x] at LineTerminator that rules out a restricted production
• [x] but not as empty statement
• [x] and not in for(;;) loop head
• [x] not even in for (let x = 0 \n x < 4; x++) {}.

[jorendorff]

One idea I had for jsparagus generally was the ability to mark any point in any production and say "this right here is a special position" and give the specialness a name, like:

IterationStatement ::= "do" Statement "while" "(" Expression ")" [label:magic_asi] ";"

And at run time, if an error happens, such states get a special error handler. This feature would fit the theme of jsparagus being "the hackable LALR parser generator".

However, two caveats:

• Maybe yacc-styleerror productions are better.

• It's not clear how general this mechanism would end up being. I go back and forth between only wanting it for do-while and thinking it could solve all of ASI.

[jorendorff]

Error productions are better, can be trivially generated from the standard grammar, and can implement all the ASI quirks except "[no LineTerminator here]" and for (let x=0 x<n; x++).

[jorendorff]

Ticked the last checkbox in #128.