Fix parser strictMode option - #13548

The strictMode option to parse() does not behave as described in the docs.

The docs say:

strictMode: By default, ECMAScript code is parsed as strict only if a "use strict"; directive is present or if the parsed file is an ECMAScript module. Set this option to true to always parse files in strict mode.

The actual behavior is strict = options.strictMode === false ? false : options.sourceType === "module";

Setting the option to true has no effect - contradicting what it says in docs.

This PR fixes this so the option behaves as documented, while also preserving the current (undocumented) behavior of allowing strictMode: false to override sourceType: 'module'.

I have opted to check for true and false explicitly and ignore any other values for the option, rather than accepting other truthy/falsy values. This is intended to make this change less likely to cause unexpected breakage for incorrect configurations e.g. strictMode: "sloppy". But I think anyone who has explicitly said strictMode: true clearly wants that!

By the way, this option is useful in some cases. I ran into it because was trying to parse code that was being run in an eval() which is nested within strict mode code. So it should be parsed as strict mode, but not as a module as there's no import inside eval().