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/terminal_benchmark.gd

@@ -0,0 +1,31 @@
+extends "res://examples/terminal/terminal.gd"
+
+signal started
+signal exited(exit_code: int)
+
+var vtebench_dir := ProjectSettings.globalize_path("res://benchmark/vtebench")
+
+
+func _ready():
+	pty.connect("exited", self._on_exit)
+
+
+func run_benchmark(benchmark):
+	pty.fork(
+		"cargo",
+		["run", "--", "-b", "benchmarks/%s" % benchmark, "--dat", "../results/%s.dat" % benchmark],
+		vtebench_dir,
+		87,
+		29
+	)
+
+
+func _on_exit(exit_code, _signal):
+	exited.emit(exit_code)
+
+
+func _on_pty_data_received(data: PackedByteArray):
+	# Listen for the reset sequence (\x1bc), to determine that the benchmark has started.
+	if data.slice(0, 2) == PackedByteArray([27, 99]):
+		$PTY.disconnect("data_received", _on_pty_data_received)
+		started.emit()