Add redundant-return diagnostic

[Cr4xy]

This patch adds a diagnostic for unnecessary return statements.

Examples:

[sumneko]

Please add tests here: https://github.com/sumneko/lua-language-server/blob/master/test/diagnostics/init.lua
Including positive and negative examples

[sumneko]

I don't think that return nil is a redundant return value. Not returning any value is different from returning a nil in Lua.
Please consider the following:

function f() if XXX then return something end return nil end

function f() if XXX then return something end return nil, 'errormessage' end

function f() return a, nil, b end

[Cr4xy]

This is how it currently looks:

I can change it to only mark a 'return' without any return values:

[Cr4xy]

Is this ok? I will try to add the tests later today.

[sumneko]

Yes, thank you!