Support LuaJIT v3 syntax backported to v2.1

[vaisest]

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Windows

What is the issue affecting?

Diagnostics/Syntax Checking

Expected Behaviour

LuaJIT v3 is adding new syntax: LuaJIT/LuaJIT#1092

This is not yet very relevant, but parts of the syntax was already backported to v2.1: LuaJIT/LuaJIT#1476

LuaJIT uses a rolling release, which means this will slowly become a bigger problem. I believe most of the v3 features will not be backported, but these were due to it being required for LuaJIT development. I couldn't find any issues about it here, and it seems like the syntax isn't accepted in VSCode.

Here's a list of the currently backported extensions:

Backported extensions:

• Bit Operators: unary ~, binary & | ~ << >> ~>>
• Customary Operators ! && || !=
• Ternary ?: conditional operator
• Safe Navigation Operator ?.
• nil-Coalescing Operator ??
• Compound Assignment Operators += -= *= …
• continue Statement
• const Declaration
• Short Function Expression
• Underscores in Number Literals

Actual Behaviour

Unexpected symbol

Reproduction steps

See attached PRs.

Additional Notes

No response

Log File

No response

[Hedwig7s]

In addition, some style warnings I'd recommend:
Disallow missing spaces in ternaries with function calls for the default path (e.g. foo ? bar:baz() due to looking like a : function call
Opinionated: Disallow C-style logic operators (!, && and ||, maybe !=) for consistency and readability

[CppCXY]

In our private talks, we felt that the ternary operator's syntax would clash too heavily with Lua's method-call colon, making it less suitable for the current LuaLS parser implementation.

Additionally, since I have decided not to update emmyluacodestyle anymore, the associated formatting and style checking will no longer be supported. For formatting, please use emmylua_formatter.