Split: Fix instantiations with unions and add strictLiteralChecks option

[som-sm]

This PR introduces a new strictLiteralChecks option for the Split type based on the discussions made in #1047.

This option currently defaults to false for backward compatibility but in the next major release it'd be better to update the default to true.

Why is defaulting to true better?
When non-literal strings are involved, the result produced by Split will most-likely be inaccurate. Consider this example:

import {Split} from "type-fest";

declare function split<
	S extends string,
	Delimiter extends string,
>(string: S, separator: Delimiter): Split<S, Delimiter, {strictLiteralChecks: false}>;

const result = split('a.b.c.d' as `a.b.${string}`, '.');
//    ^? const result: ["a", "b", string]

So, when strictLiteralChecks is disabled, splitting a.b.${string} by '.' returns ['a', string, 'z'], which is not accurate because the a.b.${string} type might hold a value like a.b.c.d, which, when split by ,, would result in ['a', 'b', 'c', 'd'], which wouldn't conform to the output type of ['a', 'b', string].

So, by default, it's better to prioritise accuracy over precision and simply return string[] in cases involving non-string literals.

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This PR also fixes cases where Delimiter is a union:

type Current = Split<"a,b,c", "," | "b">;
//   ^? type Current = ["a" | "a,", "" | "b", "", "c"] | ["a" | "a,", "" | "b", "c"] | ["a" | "a,", "", "c"]

type Updated = Split<"a,b,c", "," | "b">;
//   ^? type Updated = ["a", "b", "c"] | ["a,", ",c"]

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Closes #1047