Flutter_flags: Bug fixing

[ByJerome]

• Issue with JP flag rendering, along some others, in 6.0.0 #70 removed almost all clipPath-tags inside the defs-tag (Flags were cut off)
• FormatException("Invalid double", source) #77 FormatException with Malta 4x3 svg #71 changed the pt values to pixel (1pt ~= 1.3333333px)
• Exception when drawing Peru flag #69 Argentina's flag issue #74 [Version: 6.0.0] Failed to find definition for #a #67 Flutter svg goes through the ids from top to bottom. In some svg's there is first a href="url(#a)" link to an id
  which is set afterwards with id="a". I added the required svg elements to the defs tag for the flags where the error occurred. I
  did NOTHING to the flags themselves except the Korean flag because it was broken.
• Added around 60 missing flags into the FlagsCode Enum
• Updated flutter_svg to version 1.0.3
• Updated the flags (6.6.3) provided by https://github.com/lipis/flag-icons

All flags are working without any errors either on 4x3 or 1x1 format. & nearly all issues are resolved with this pr.

This is the first pull request I have ever made. Be kind with me 😄

[JulianBissekkou]

Nice! @LunaGao Can you give this a review?

[daniel-hopkins]

I see this merge, but the version on pub.dev has not been updated.
Is that correct?

[mahanabd]

why the library not update flutter_svg?

[stefanschaller]

Any information why flutter_svg are not up to date?

[LunaGao]

Updated.