Add AxisClick event

[bryevdv]

This PR adds support for an AxisClick / "axis_click" event that reports the location that an axis was clicked.

• issues: fixes Add support for click events on categorical axes #601
• tests added / passed
• release document entry (if new feature or API change)

This can be tested by adding code like the following to nearly any example":

from bokeh.models import CustomJS
p.xaxis[0].js_on_event("axis_click", CustomJS(code="""
    alert('It works! ' + cb_obj.value)
"""))

p.yaxis[0].js_on_event("axis_click", CustomJS(code="""
    alert('It works! ' + cb_obj.value)
"""))

This needs some polish:

• @mattpap as usual any suggestions for BokehJS tests welcome. I did not see much existing tests for these sorts of events that I might look to emulate
• @bokeh/dev thoughts on interesting examples? This has been long-requested but there were never concrete use-cases described that I recall.
• @tcmetzger any thoughts on where / how much narrative docs to add are welcome

[mattpap]

as usual any suggestions for BokehJS tests welcome. I did not see much existing tests for these sorts of events that I might look to emulate

There aren't many tests for this, but there is one recently added that should be quite useful as a basis:

bokeh/bokehjs/test/unit/regressions.ts

Lines 663 to 692 in 78dcbf3

	describe("in issue #12778", () => {
	it("doesn't allow emitting RangesUpdate event for linked plots", async () => {
	const p0 = fig([200, 200], {tools: "pan"})
	const p1 = fig([200, 200], {tools: "pan", x_range: p0.x_range, y_range: p0.y_range})
	const p2 = fig([200, 200], {tools: "pan", x_range: p0.x_range})
	p0.scatter([1, 2, 3], [0, 1, 2])
	p1.scatter([1, 2, 3], [2, 3, 4])
	p2.scatter([1, 2, 3], [5, 6, 7])
	const events: RangesUpdate[] = []
	p0.on_event(RangesUpdate, (event) => events.push(event))
	p1.on_event(RangesUpdate, (event) => events.push(event))
	p2.on_event(RangesUpdate, (event) => events.push(event))
	const row = new Row({children: [p0, p1, p2]})
	const {view} = await display(row)
	const pv0 = view.owner.get_one(p0)
	const actions = new PlotActions(pv0)
	await actions.pan(xy(2, 1), xy(2, 3))
	await paint()
	expect(events.length).to.be.equal(3)
	expect(events[0].origin).to.be.equal(p0)
	expect(events[1].origin).to.be.equal(p1)
	expect(events[2].origin).to.be.equal(p2)
	})
	})

[bryevdv]

@mattpap it looks like you've addressed all your own comments?

[mattpap]

it looks like you've addressed all your own comments?

Yes, given that they were all related to the same code block.

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