In IE (all versions), `focus` & `blur` handlers are fired asynchronously
but `focusin` & `focusout` are run synchronously. In other browsers, all
those handlers are fired synchronously. Asynchronous behavior of these
handlers in IE caused issues for IE (gh-4856, gh-4859).

We now simulate `focus` via `focusin` & `blur` via `focusout` in IE to avoid
these issues. This also let us simplify some tests.

This commit also simplifies `leverageNative` - with IE now using `focusin`
to simulate `focus` and `focusout` to simulate `blur`, we don't have to deal
with async events in `leverageNative`. This also fixes broken `focus` triggers
after first triggering it on a hidden element - previously, `leverageNative`
assumed that the native `focus` handler not firing after calling the native 
`focus` method meant it would be handled later, asynchronously, which
was not the case (gh-4950).

To preserve relative `focusin`/`focus` & `focusout`/`blur` event order
guaranteed on the 3.x branch, attach a single handler for both events in IE.

A side effect of this is that to reduce size the `event/focusin` module
no longer exists and it's impossible to disable the `focusin` patch
in modern browsers via the jQuery custom build system.

Fixes gh-4856
Fixes gh-4859
Fixes gh-4950
Ref gh-5223
Closes gh-5224

Co-authored-by: Richard Gibson <[email protected]>

Previously, when `leverageNative` handled async events, there was
a case where an empty placeholder object was set as a result.
Covering both such an object and `false` required a `length` check.
However, this is not necessary since jquerygh-5223 and the check was
already simplified in other places; this one was missed.

Ref jquerygh-5223

Previously, when `leverageNative` handled async events, there was
a case where an empty placeholder object was set as a result.
Covering both such an object and `false` required a `length` check.
However, this is not necessary since gh-5223 and the check was
already simplified in other places; this one was missed.

Closes gh-5236
Ref gh-5223

Previously, when `leverageNative` handled async events, there was
a case where an empty placeholder object was set as a result.
Covering both such an object and `false` required a `length` check.
However, this is not necessary since gh-5223 and the check was
already simplified in other places; this one was missed.

Closes gh-5236
Ref gh-5223

(cherry picked from commit dfe212d)