Fixes split to return non-null first element in most cases.

Fixes microsoft#49635
Behavior similar to microsoft#49682

microsoft#49635 was originally rejected because JavaScript has a stupid edge case where `''.split('')` and `''.split(new RegExp(''))` return an empty array so we cannot assert that the first element is *always* a string.  However, given that the vast majority of use cases for `split` include a literal separator, we can create a set of overloads that results in the non-empty literal separator returning a more narrow type that has a string for the first element.

```ts
''.split('') // []; type= string[]
''.split(new RegExp('')) // []; type= string[]
''.split('a') // ['']; type= [string, ...string[]]
''.split(new RegExp('a')) // ['']; type= string[]
```

Unfortunately, I don't know of a way to differentiate a regular expression literal and a regular expression so that case still always returns an empty first element even though I don't even think it is possible to have an empty regular expression literal.

I don't *believe* there are any other edge cases where split can return an empty array, but if there are please let me know and I can try to update, or we can close the PR and just leave a comment on microsoft#49635 for the next person.

Author: MicahZoltu

src/lib/es5.d.ts

@@ -470,6 +470,8 @@ interface String {
      * @param separator A string that identifies character or characters to use in separating the string. If omitted, a single-element array containing the entire string is returned.
      * @param limit A value used to limit the number of elements returned in the array.
      */
+    split(separator: '', limit?: number): string[];
+    split(separator: string, limit?: number): [string, ...string[]];
     split(separator: string | RegExp, limit?: number): string[];
 
     /**