font: add is_char_defined method in Font and Freetype

[dr0id]

partial PR as requested in this PR #2132

[yunline]

import pygame
import pygame.font

pygame.font.init()

font=pygame.font.Font(size=30)
is_defined=lambda char:print("%s:"%repr(char),font.is_char_defined(char))

is_defined("a")
is_defined("\n")
is_defined("锟")
is_defined("斤")
is_defined("拷")
is_defined(chr(0x20001))

'a': True
'\n': False
'锟': False
'斤': False
'拷': False
Traceback (most recent call last):
  File "x:\python\pygame_test\is_char_defined.py", line 14, in <module>
    is_defined(chr(0x20001))
  File "x:\python\pygame_test\is_char_defined.py", line 7, in <lambda>
    is_defined=lambda char:print("%s:"%repr(char),font.is_char_defined(char))
ValueError: Too long, only 1 char supported.

I passed only one char but got a "Too long, only 1 char supported."
It seems that characters above 0xffff is not handled correctly.
TTF_GlyphIsProvided32 is recommended except SDL_ttf<2.0.18, TTF_GlyphIsProvided can be a fall back.