Fix String case functions inconsistency

[AlexGx]

Make String.upcase/2, String.downcase/2, and String.capitalize/2
consistently reject invalid modes when an empty or a single grapheme is passed.

Before

iex> String.capitalize("a", :invalid)
"A"
# expected: **(FunctionClauseError) no function clause matching in String.capitalize/2  

iex> String.upcase("", :invalid)
""
# expected: ** (FunctionClauseError) no function clause matching in String.upcase/2 

iex> String.capitalize("aa", :invalid)
** (FunctionClauseError) no function clause matching in String.downcase/2
# expected more verbose: ** (FunctionClauseError) no function clause matching in String.capitalize/2

After

Unsupported modes consistently fail as expected by public boundary

Inspired by #15585

[josevalim]

I really think those cases will be best caught by the type system. The float made sense because the confusion between 0 and 0.0 is more subtle but here the type system will do a fine job. Thanks!