Add `upvalue` semantic token modifier

The issue names no files or tests. Start by locating LuaLS semantic-token generation and the lexical-scope resolution used for variable references; trace how the existing global modifier is assigned. Done means references resolved to enclosing-scope variables receive an upvalue modifier, declarations and ordinary locals remain unchanged, and nested closures are covered by tests.

LuaLS should expose whether a variable reference is an upvalue through a new upvalue semantic token modifier.

Currently, local variables and upvalues are both reported as:

semantic token type: variable
modifiers:           -

while globals are already distinguished using:

semantic token type: variable
modifiers:           global

This makes it impossible for editors such as VS Code to visually distinguish local variables from upvalues.

Example

local function outer()
    local value = 123

    return function()
        print(value) -- upvalue
    end
end

Currently the value reference inside the inner function is:

variable

It would be useful for LuaLS to report it as:

variable + upvalue

while the declaration remains:

variable

Proposed semantic token model

variable           -> local variable
variable + upvalue -> upvalue
variable + global  -> global variable
parameter          -> function parameter

For example:

local foo = 1

local function outer(bar)
    local baz = 2

    return function(qux)
        print(foo)    -- upvalue
        print(bar)    -- upvalue
        print(baz)    -- upvalue
        print(qux)    -- parameter
        print(print)  -- global
    end
end

would produce approximately:

foo   -> variable + upvalue
bar   -> variable + upvalue
baz   -> variable + upvalue
qux   -> parameter
print -> variable + global

Ordinary locals would remain simply:

variable

Why a modifier?

A modifier seems preferable to introducing a new semantic token type because upvalue describes a property of a variable reference rather than a fundamentally different kind of symbol.

It also fits the existing LuaLS representation of globals:

variable + global

This gives editor themes a simple way to distinguish all four cases:

variable
variable + upvalue
variable + global
parameter

Scope

The upvalue modifier should be applied to references resolved to a variable declared in an enclosing lexical scope.

For example:

local x = 1

local function f()
    print(x)
end

would be:

x declaration -> variable
x reference   -> variable + upvalue

Nested closures should work recursively.

Motivation

This would allow VS Code themes to use different colors for locals, parameters, upvalues, and globals, making lexical scope much easier to understand in closure-heavy Lua/LuaJIT code.

LuaLS already performs the necessary lexical-scope resolution; this proposal would expose that information through semantic tokens.