docs(readme): add chaining documentation

build: add package script for running just locator tests

fix(fixture): support function `TextMatch` argument in queries

docs(readme): add `Locator` documentation and clarify different APIs

feat(fixture): support `Page` in `within` fixture (returns `Screen`)

Support `Page` in addition to `Locator` in `within()` fixture. When
a `Page` instance is passed to `within()`, a `Screen` instance will be
returned which includes the entire `Page` API in addition to the query
methods.

```ts
test('within page', async ({ page }) => {
  // Passing `page` to `within` returns a `Screen` instance
  const screen = within(page)

  await screen.goto(/* ... */)

  const form = screen.queryByRole('form', {name: 'User'})

  // Passing a `Locator` returns scoped queries
  const {queryByLabelText} = within(form)

  // ...
})
```

fix(fixture): throw correct error when `find*` query times out on vis…

…ibility

docs: add inline documentation for `configure`

docs: add inline documentation for `configure`

test: oops, don't run duplicate test suites

refactor: derive configuration options type for Testing Library types