Improve the os.difftime type definition

[tapple]

os.difftime cannot accept DateTypeResults:

--!strict
local t1 = os.date("!*t")
local t2 = os.date("!*t")
print(os.difftime(t1, t2))
print(t1 - t2)

$ ./luau osbug.luau 
./osbug.luau:4: invalid argument #2 to 'difftime' (number expected, got table)
stacktrace:
[C] function difftime
./osbug.luau:4

But luau-analyze thought it could:

$ ./luau-analyze osbug.luau 
./osbug.luau(5,7): TypeError: Binary operator '-' not supported by types 'DateTypeResult' and 'DateTypeResult'

Fixed:

$ ./luau-analyze osbug.luau 
./osbug.luau(4,19): TypeError: Expected this to be 'number', but got 'DateTypeResult'
./osbug.luau(4,23): TypeError: Expected this to be 'number', but got 'DateTypeResult'
./osbug.luau(5,7): TypeError: Binary operator '-' not supported by types 'DateTypeResult' and 'DateTypeResult'