cli: enable can_block by default, add --no-can-block flag

Make boa's cli consistent with the behaviour of other major JavaScript
shells (d8, jsc, spidermonkey at least) as well as boa_tester by
enabling [[CanBlock]] by default.

Add --no-can-block flag to opt out, same as V8
(https://github.com/v8/v8/blob/main/src/d8/d8.cc#L6417).

Needed for Atomics tests to pass when using an external test262 harness.

Author: ivankra

cli/src/main.rs

@@ -157,6 +157,10 @@ struct Opt {
     #[arg(long)]
     test262_object: bool,
 
+    /// Disallow the main thread from blocking (e.g. `Atomics.wait`).
+    #[arg(long)]
+    no_can_block: bool,
+
     /// Treats the input files as modules.
     #[arg(long, short = 'm', group = "mod")]
     module: bool,
@@ -558,6 +562,7 @@ fn main() -> Result<()> {
     let context = &mut ContextBuilder::new()
         .job_executor(executor.clone())
         .module_loader(loader.clone())
+        .can_block(!args.no_can_block)
         .build()
         .map_err(|e| eyre!(e.to_string()))?;