Add ruby predefined objects to variable name checker

[styd]

Hi, @yuki24.
I'm thinking of adding predefined objects (false, true, and nil) in the variable name checker dictionary. Their 2.4 deprecated toplevel constants are already included, but the objects themselves are not.
The constants:

a = NOL
# NameError: uninitialized constant NOL
# Did you mean?  NIL
b = FALS
# NameError: uninitialized constant FALS
# Did you mean?  FALSE
c = TREU
# NameError: uninitialized constant TREU
# Did you mean?  TRUE

but, the objects:

a = nol
# NameError: undefined local variable or method `nol' for main:Object
b = fals
# NameError: undefined local variable or method `fals' for main:Object
c = treu
# NameError: undefined local variable or method `treu' for main:Object

I need to be reminded of these objects too.

I don't know how to get the predefined objects in the nice way so I just write them in an array.
Since nil return "" when called with to_s, I replaced it with :nil.
Should I wrote all of them in symbols ([:false, :true, :nil]) to make it consistent with the others?

If this PR is merged:

a = nol
# NameError: undefined local variable or method `nol' for main:Object
# Did you mean?  nil
b = fals
# NameError: undefined local variable or method `fals' for main:Object
# Did you mean?  false
c = treu
# NameError: undefined local variable or method `treu' for main:Object
# Did you mean?  true

[yuki24]

This is a great addition. Thanks!