feat(bench): add BenchContext::start() and BenchContext::end() (#18734)

Closes #17589.
```ts
Deno.bench("foo", async (t) => {
  const resource = setup(); // not included in measurement
  t.start();
  measuredOperation(resource);
  t.end();
  resource.close(); // not included in measurement
});
```

1 files changed, 50 insertions, 0 deletions

diff --git a/cli/tests/testdata/bench/explicit_start_and_end.ts b/cli/tests/testdata/bench/explicit_start_and_end.ts
new file mode 100644
index 000000000..60a3d10d7
--- /dev/null
+++ b/cli/tests/testdata/bench/explicit_start_and_end.ts
@@ -0,0 +1,50 @@
+Deno.bench("start and end", (t) => {
+  const id = setInterval(() => {}, 1000);
+  t.start();
+  Deno.inspect(id);
+  t.end();
+  clearInterval(id);
+});
+
+Deno.bench("start only", (t) => {
+  const id = setInterval(() => {}, 1000);
+  t.start();
+  Deno.inspect(id);
+  clearInterval(id);
+});
+
+Deno.bench("end only", (t) => {
+  const id = setInterval(() => {}, 1000);
+  Deno.inspect(id);
+  t.end();
+  clearInterval(id);
+});
+
+Deno.bench("double start", (t) => {
+  const id = setInterval(() => {}, 1000);
+  t.start();
+  t.start();
+  Deno.inspect(id);
+  t.end();
+  clearInterval(id);
+});
+
+let captured: Deno.BenchContext;
+
+Deno.bench("double end", (t) => {
+  captured = t;
+  const id = setInterval(() => {}, 1000);
+  t.start();
+  Deno.inspect(id);
+  t.end();
+  t.end();
+  clearInterval(id);
+});
+
+Deno.bench("captured", () => {
+  const id = setInterval(() => {}, 1000);
+  captured!.start();
+  Deno.inspect(id);
+  captured!.end();
+  clearInterval(id);
+});