Use symbol for rest property in grammars

[RunDevelopment]

Motivation
Prism supports a rest property in grammars that essentially functions as an object spread replacement. The behavior of rest and object spread is subtly different, so we can't remove support for rest, unfortunately. However, we may as well fix the worst aspect of rest: it gets in the way of token iteration.

Everything that processes or analyses grammars has to take rest into account, because rest is the only property of a grammar that isn't a token (this also creates problems with TS).

Description
Replace the rest property with a symbol, let's call it Prism.rest.

This will solve the iteration problem, because symbols are non-enumerable by default.

This will also make it possible to finally define a proper type for Prism grammars, which simply wasn't possible before.

interface Grammar {
  [key: string]: RegExp | GrammarToken | Array<RegExp | GrammarToken>;
  [Prism.rest]?: Grammar;
}

[RunDevelopment]

Implemented in v2. The only difference is that the rest symbol is exported using module exports and not as a property of Prism instances.