Add string-case types

[KajSzy]

This solves #16

type Union = "a" | "b" | "c";

const foo: Union = "a";

const fooUpperCased: Uppercase<Union> = foo.toUpperCase(); // should be 'A' | 'B' | 'C'

[helmturner]

I can't think of another language with upper- and lower-case letters to throw at it, but it's passing these tests (basically the same tests you wrote, but consolidated and made more exhaustive).

type EnglishLower = "abcdefghijklmnopqrstuvwxyz"
type EnglishUpper = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
type CyrillicLower = "юабцдефгхийклмнопярстужвьызшэщчъ"
type CyrillicUpper = "ЮАБЦДЕФГХИЙКЛМНОПЯРСТУЖВЬЫЗШЭЩЧЪ"

doNotExecute(async () => {  
  doNotExecute(async () => {
    const englishUpperString = "" as EnglishUpper;
    const cyrillicUpperString = "" as CyrillicUpper;
  
    const englishLowerString = englishUpperString.toLowerCase();
    const cyrillicLowerString = cyrillicUpperString.toLowerCase();

    const englishLocaleLowerString = englishUpperString.toLocaleLowerCase();
    const cyrillicLocaleLowerString = cyrillicUpperString.toLocaleLowerCase();

    type tests = [
        Expect<Equal<typeof englishLowerString, EnglishLower>>,
        Expect<Equal<typeof cyrillicLowerString, CyrillicLower>>,
        Expect<Equal<typeof englishLocaleLowerString, EnglishLower>>,
        Expect<Equal<typeof cyrillicLocaleLowerString, CyrillicLower>>,

        Expect<NotEqual<typeof englishUpperString, typeof englishLowerString>>,
        Expect<NotEqual<typeof cyrillicUpperString, typeof cyrillicLowerString>>,
        Expect<NotEqual<typeof englishUpperString, typeof englishLocaleLowerString>>,
        Expect<NotEqual<typeof cyrillicUpperString, typeof cyrillicLocaleLowerString>>,
    ];
  });
});

  doNotExecute(async () => {
    const englishLowerString = "" as EnglishLower;
    const cyrillicLowerString = "" as CyrillicLower;
  
    const englishUpperString = englishLowerString.toUpperCase();
    const cyrillicUpperString = cyrillicLowerString.toUpperCase();

    const englishLocaleUpperString = englishLowerString.toLocaleUpperCase();
    const cyrillicLocaleUpperString = cyrillicLowerString.toLocaleUpperCase();

    type tests = [
        Expect<Equal<typeof englishUpperString, EnglishUpper>>,
        Expect<Equal<typeof cyrillicUpperString, CyrillicUpper>>,
        Expect<Equal<typeof englishLocaleUpperString, EnglishUpper>>,
        Expect<Equal<typeof cyrillicLocaleUpperString, CyrillicUpper>>,

        Expect<NotEqual<typeof englishLowerString, typeof englishUpperString>>,
        Expect<NotEqual<typeof cyrillicLowerString, typeof cyrillicUpperString>>,
        Expect<NotEqual<typeof englishLowerString, typeof englishLocaleUpperString>>,
        Expect<NotEqual<typeof cyrillicLowerString, typeof cyrillicLocaleUpperString>>,
    ];
  });