feat(cli): add DENO_V8_FLAGS env var (#17313)

Closes #5669

cli/util/v8.rs

@@ -0,0 +1,47 @@
+// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
+
+#[inline(always)]
+pub fn get_v8_flags_from_env() -> Vec<String> {
+  std::env::var("DENO_V8_FLAGS")
+    .ok()
+    .map(|flags| flags.split(',').map(String::from).collect::<Vec<String>>())
+    .unwrap_or_default()
+}
+
+#[inline(always)]
+pub fn construct_v8_flags(
+  v8_flags: &[String],
+  env_v8_flags: Vec<String>,
+) -> Vec<String> {
+  std::iter::once("UNUSED_BUT_NECESSARY_ARG0".to_owned())
+    .chain(env_v8_flags.into_iter())
+    .chain(v8_flags.iter().cloned())
+    .collect::<Vec<_>>()
+}
+
+pub fn init_v8_flags(v8_flags: &[String], env_v8_flags: Vec<String>) {
+  if v8_flags.is_empty() && env_v8_flags.is_empty() {
+    return;
+  }
+
+  let v8_flags_includes_help = env_v8_flags
+    .iter()
+    .chain(v8_flags)
+    .any(|flag| flag == "-help" || flag == "--help");
+  // Keep in sync with `standalone.rs`.
+  let v8_flags = construct_v8_flags(v8_flags, env_v8_flags);
+  let unrecognized_v8_flags = deno_core::v8_set_flags(v8_flags)
+    .into_iter()
+    .skip(1)
+    .collect::<Vec<_>>();
+  if !unrecognized_v8_flags.is_empty() {
+    for f in unrecognized_v8_flags {
+      eprintln!("error: V8 did not recognize flag '{}'", f);
+    }
+    eprintln!("\nFor a list of V8 flags, use '--v8-flags=--help'");
+    std::process::exit(1);
+  }
+  if v8_flags_includes_help {
+    std::process::exit(0);
+  }
+}