HTML Reporter: Convert to HtmlReporter class

=== What ===

Turn html.js into an HtmlReporter class where the HtmlReporter.init
static method contains (most) of the browser runner logic that we
would run even if the HtmlReporter were turned off.

This patch primarily deals with making html.js itself free of
side-effects by delaying any browser setup logic to the init()
function.

In a future patch we can take this further and move the logic to a
"browser runner" function of sorts, such that what is left in the
reporter can be made conditional on the presence of `div[id=qunit]`
and/or `QUnit.config.reporters.html`. This is tracked in
qunitjs#1118 and
qunitjs#1711.

Ref qunitjs#1486.

=== Why ===

We need html.js to be side-effect free in order to add native ESM
support, tracked at qunitjs#1551.

To support native ESM exports, we need to introduce a separate
distribution file since ESM export syntax isn't allowed in scripts.
I expect numerous projects to involve mixed ESM imports and CJS require
usage, especially when integration layers and plugins are involved.

To avoid a split-brain problem, this means we need to somehow have
both export the same one. The way I intend to do this is by detecting
if QUnit was already defined, and if so, discard our local definition
and use that one instead. This means we need to only init the browser
runner only if we're the first one (e.g. window.onerror, QUnit.begin
for fixture, QUnit.testDone for HTML reporting, etc).

To accomplish that, start by making html.js side-effect free, deferring
its init code to a (for now) unconditional call at the end of qunit.js.