perf(term): add benchmarks

Adds benchmarks by running [alacritty/vtebench](https://github.com/alacritty/vtebench)
benchmarks in the terminal.
Uses code based on [godotengine/godot-benchmarks](https://github.com/godotengine/godot-benchmarks)
to measure average GPU and CPU time spent per frame.
Uses [github-action-benchmark](https://github.com/benchmark-action/github-action-benchmark)
for continuous integration, and publishes benchmark results to https://lihop.github.io/godot-xterm/dev/bench/.

benchmark/benchmark.gd

@@ -0,0 +1,124 @@
+extends Control
+
+
+class Results:
+	var render_cpu := 0.0
+	var render_gpu := 0.0
+	var vtebench := {value = 0.0, variance = 0.0}
+
+
+var terminal_exit_code := -1
+
+
+func _ready():
+	var timeout := 120
+	var benchmark := ""
+
+	var args = OS.get_cmdline_user_args()
+	for arg in args:
+		if arg.begins_with("--benchmark"):
+			benchmark = arg.split("=")[1]
+
+	if benchmark.is_empty():
+		_quit_with_error("No benchmark specified")
+
+	RenderingServer.viewport_set_measure_render_time(get_tree().root.get_viewport_rid(), true)
+
+	var results := Results.new()
+	var begin_time := Time.get_ticks_usec()
+	var frames_captured := 0
+
+	$Terminal.run_benchmark(benchmark)
+	await $Terminal.started
+
+	while terminal_exit_code == -1:
+		await get_tree().process_frame
+
+		if Time.get_ticks_usec() - begin_time > (timeout * 1e6):
+			_quit_with_error("Benchmark took longer than %ss to run" % timeout)
+
+		results.render_cpu += (
+			RenderingServer.viewport_get_measured_render_time_cpu(
+				get_tree().root.get_viewport_rid()
+			)
+			+ RenderingServer.get_frame_setup_time_cpu()
+		)
+		results.render_gpu += RenderingServer.viewport_get_measured_render_time_gpu(
+			get_tree().root.get_viewport_rid()
+		)
+
+	if terminal_exit_code != 0:
+		_quit_with_error("Terminal exited with error code: %d" % terminal_exit_code)
+
+	results.render_cpu /= float(max(1.0, float(frames_captured)))
+	results.render_gpu /= float(max(1.0, float(frames_captured)))
+
+	results.vtebench = _process_dat_results("res://benchmark/results/%s.dat" % benchmark)
+
+	var json_results = (
+		JSON
+		. stringify(
+			[
+				{
+					name = benchmark,
+					unit = "milliseconds",
+					value = _round(results.vtebench.value),
+					range = results.vtebench.range,
+				},
+				{
+					name = "%s - render cpu" % benchmark,
+					unit = "milliseconds",
+					value = _round(results.render_cpu),
+				},
+				{
+					name = "%s - render gpu" % benchmark,
+					unit = "milliseconds",
+					value = _round(results.render_gpu),
+				}
+			],
+			"    "
+		)
+	)
+
+	var file = FileAccess.open("res://benchmark/results/%s.json" % benchmark, FileAccess.WRITE)
+	file.store_string(json_results)
+
+	print(json_results)
+	get_tree().quit(terminal_exit_code)
+
+
+func _on_terminal_exited(exit_code: int):
+	terminal_exit_code = exit_code
+
+
+func _round(val: float, sig_figs := 4) -> float:
+	return snapped(val, pow(10, floor(log(val) / log(10)) - sig_figs + 1))
+
+
+func _process_dat_results(path: String) -> Dictionary:
+	var file := FileAccess.open(path, FileAccess.READ)
+	var samples := []
+
+	file.get_line()  # Skip the first 'header' line.
+	while !file.eof_reached():
+		var line := file.get_line().strip_edges()
+		if line.is_valid_float():
+			samples.append(line.to_float())
+
+	if samples.size() < 2:
+		_quit_with_error("Not enough samples")
+
+	var avg: float = (samples.reduce(func(acc, n): return acc + n, 0)) / samples.size()
+
+	var std_dev := 0.0
+	for sample in samples:
+		std_dev += pow(sample - avg, 2)
+	std_dev /= (samples.size() - 1)
+
+	return {value = avg, range = "± %.2f" % _round(sqrt(std_dev))}
+
+
+func _quit_with_error(error_msg: String):
+	await get_tree().process_frame
+	push_error(error_msg)
+	get_tree().quit(1)