Add `--strict` flag to cli (#2689)

This PR adds the `--strict` flag to the CLI that enables strict mode on file/interactive mode execution. 

It's a bit annoying to have to prefix with `'use strict';`  when trying to debug in interactive mode
```js
>>> 'use strict'; .... // :(
```

It changes the following:
- Adds `--strict` flag to CLI
- update `README.md`

README.md

@@ -82,6 +82,9 @@ Arguments:
           The JavaScript file(s) to be evaluated
 
 Options:
+      --strict
+          Run in strict mode
+
   -a, --dump-ast [<FORMAT>]
           Dump the AST to stdout with the given format
 

boa_cli/src/main.rs

@@ -94,6 +94,10 @@ struct Opt {
     #[arg(name = "FILE", value_hint = ValueHint::FilePath)]
     files: Vec<PathBuf>,
 
+    /// Run in strict mode.
+    #[arg(long)]
+    strict: bool,
+
     /// Dump the AST to stdout with the given format.
     #[arg(
         long,
@@ -256,6 +260,9 @@ fn main() -> Result<(), io::Error> {
         .build()
         .expect("cannot fail with default global object");
 
+    // Strict mode
+    context.strict(args.strict);
+
     // Trace Output
     context.set_trace(args.trace);