Add ability to provide descriptor of other types

Allow Descriptors, primitive strings, regexps and functions. Other values will be added to set validator throw "unknown validation" error.

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When supplied validation is a string, it is substituted by { [validator]: validation } descriptor (there has to be a validation-to-validator(s) map):

valur(42).as("integer");
// same as valur(42).as({ number: "integer" });
// only "number" validator has "integer" validation

valur(42).as("number");
// same as valur(42).as({ typeOf: "number" });
// only "typeOf" validator has "number" validation

When supplied validation is function, it is substituted by { custom: validation } descriptor:

valur(42).as(v => !(v % 1));
// same as valur(42).as({ custom: v => !(v % 1) });

valur(42).as(RegExp);
// same as valur(42).as({ custom: RegExp });
// there is no way at the moment to definitively tell classes from plain functions 🙁

When supplied validation is a regular expression, it is substituted by { pattern: validation } descriptor:

valur("text").as(/regex/);
// same as valur("text").as({ pattern: /regex/ });

Other values will throw "unknown validation" error:

valur(42).as("nubmer");
// Unknown validation: <string> nubmer