Add Grammar documentation

[JohnnyMorganz]

This PR adds grammar documentation to the webpage (closes #148)

I got the initial definitions (available in the initial commit) from the comments in https://github.com/Roblox/luau/blob/master/Ast/src/Parser.cpp.

Subsequent commits deal with including definitions which were missing, fixing incorrect definitions, and tidying up the definitions. I've tried to break it up into small commits so you can see the changes I made against the initial definitions in the Parser.cpp file - you may want the comments updated in Parser.cpp in a different PR.

A few things of note:

• bindinglist is a strange one, it has the option to allow 3 dots depending on what we are parsing, which isn't something I can express in EBNF (to my knowledge). I was kinda confused what the 3 dots were about, I think its for the ... vararg in a parameter list? but thats also described in parlist, so I'm not entirely sure. I took the bindinglist definition from the comments in Parser.cpp, but it is also used in local declarations and for loops, so it doesn't really fit right now.
• Bit confused on what to do with functioncall, it seems to be a subpart of primaryexp, but primaryexp also contains normally indexing. I wasn't sure whether to break it down like in Lua5.1/5.4, as I didn't want to deviate from how it works internally.
• Theres a few inconsistencies around the place, exp or expr? STRING or String for terminal nodes? all lower case vs. PascalCase for definitions (PascalCase seems to only be used for type-related definitions?). I've normalised (most) of them to be consistent, but maybe you want it a bit different.

I've worked my way through Parser.cpp to come up with the document, but I'm bound to have missed something. Let me know if there's a mistake somewhere!

[zeux]

We should rename TypeAnnotation to Type (incl SimpleTA etc.); we used to use TypeAnnotation in the AST names but it's actually a bit misleading because "annotation" refers specifically to ": Type" syntax, e.g. in local a: number & string the sequence : number & string is a type annotation, but number & string is a type (just as number and string)

[zeux]

This should also include the more recent changes wrt type packs, see https://luau-lang.org/typecheck#type-packs

[alexmccord]

Thanks! Our grammar in Parser.cpp by and large is dated and incorrect, so there's some things to tie up on.

bindinglist is Luau's version of Lua's namelist. I think it deserves its own name specifically because it has type annotations. Maybe we should add a comment? The syntax for that in EBNF is (* ... *).

I think we want to use exp specifically to be consistent with Lua's original grammar, but we do use expr very often outside of this EBNF.

I'll review the rest of this next week.

[zeux]

Thanks! I'm going to merge this as I think this is pretty accurately depicting the Luau grammar. Would be fun to get some generator that can use EBNF directly and see if we have omissions here but we can always correct issues if found later.

[JohnnyMorganz]

Thanks! I'm going to merge this as I think this is pretty accurately depicting the Luau grammar. Would be fun to get some generator that can use EBNF directly and see if we have omissions here but we can always correct issues if found later.

Just wanted to point out I haven't implemented the type pack syntax yet that you mentioned (unfortunately been busy with some other stuff rn). I don't know if you want to just merge it now anyways or wait until that's done (I need to look into it more).

[zeux]

Thanks - I think it's fine to merge this as is because we'll need tweaks to generic arguments anyway following the newer updates for default values. Maybe @vegorov-rbx can help with this? In the meantime I've added a note to the page that documents the omission.