Add support for Lua 5.4 syntax

[GavinHigham]

Lua 5.4 replaces namelist with attnamelist.

In Lua 5.3, one possible statement ("stat") is local namelist [‘=’ explist] .
In Lua 5.4 this is now local attnamelist [‘=’ explist].

In Lua 5.3, "namelist" is namelist ::= Name {‘,’ Name}.
In Lua 5.4, "attnamelist" is attnamelist ::= Name attrib {‘,’ Name attrib} with
attrib ::= [‘<’ Name ‘>’].

Essentially, where before you could have a local comma-separated list of names, now you can have a local comma-separated list of names, with an optional attribute for each (to support the "const" and "close" attributes). The syntax itself does not enforce what is or isn't a valid attribute, except that it must be a terminal Name.

Before:
local apple, carrot = 'fruit', 'vegetable'
After:
local apple <const>, carrot = 'fruit', 'vegetable'