[MAJOR] rename addItemFilterFn and allow string and RegExp

[tinovyatkin]

addItemFilterFn is not the only options parameter that expects a function, but it's the only one with pseudo-typings in name. This PR is to rename it to conventional addItemFilter and allow to be RegExp instance or string to create regular expression (that will cover 90% of real life use cases I believe).

I will later submit a separate PR to use standard pattern parameter and constrains validation, but for now it's to include this breaking changes in upcoming major release.

[jshjohnson]

Using isType('Function', this.config.addItemFilter) would be consistent with the rest of the codebase. I've avoided using typeof due to this: https://bonsaiden.github.io/JavaScript-Garden/#types.typeof

[tinovyatkin]

I will submit a PR later to refactor/remove isType, etc. If you are going to TypeScript you probably will do it yourself too.

typeof together with instanceof are major type of tools TypeScript/VSCode type Intellisence relays on. They are the type guards. While they have an edge cases (like typeof new String('who is doing that?!') !== typeof String('🤷🏼‍♂️'), for simply checking that something is a function typeof fn === 'function' is flawless as well as r instanceof RegExp and the last one helps a lot to prevent further developer error with automatic type checking.

And of course it's faster here.

[tinovyatkin]

@jshjohnson by the way, here are some breaking tests for your current implementation of isType:

expect(isType("Function", async () => {})).to.equal(true);
expect(
  isType("Function", function* gen() {
    yield false;
  })
).to.equal(true);

[jshjohnson]

Fair enough - in that case it's probably worth just ditching isType