Add support for JsLIGO (experimental)

Things to do for the JsLIGO parser (among others):

• decompiler

• add support for attributes (via comments or another way?)

• fix incorrect array destructuring

• fix pretty printing

• documentation

• polymorphic + (typer)

• fix shift/reduce challenge when supporting variant with a leading |,

• fix shift/reduce challenge between bin_op(..., "<", ...) and type arguments (foo<bar>)

• add support for code injection syntax that's compatible with TypeScript syntax

  • first support verbatim strings with back ticks eg. template literals.

• fix shift/reduce challenge when parentheses for: "(" cartesian "=>" fun_type ")"

• properly separate lowercase + uppercase names as in other syntaxes

• support arrow function with () as arguments

• add support for foo = bar; (assignment to existing variable)

• fix function type arguments, requires a argument name like: type z = (a: int) => string

• support 'foo' next to "foo" for strings

• be more compliant when using {...} and ({...})

• support _ as variable name (mostly works, needs to be properly checked)

• make semicolons optional

• change sum type examples to:

type foobar = 
| ['foo' , int]
| ['bar' , int]

• fix type f = (x : int) => int

Nice to haves:

• make leading | in sum type optional
• remove necessary parentheses in (foo as x) at some point, not a blocker

Later:

• can we improve on the constructor syntax?
• make sure variable names can't be reused in the same block (ast_core)
• for-of with maps support (typer)
• export support (typer)
• constant variables support (typer)

Choices made in current implementation of the JsLIGO parser:

• pattern matching will be done via a "built-in" match function (probably implemented in the abstractor)
• "%" is used as a modulo operator, which is not correct as JS uses it for a remainder operation but close enough.
• expressions are used for function arguments as per EcmaScript specification

closes #1051 (closed) #1051 (closed)