Throw TypeError if regex is passed to String.prototype.startsWith

[jasonwilliams]

In our implementation of String.prototype.startsWith we're not type checking the argument and blindly coercing it into a string.
https://github.com/boa-dev/boa/blob/master/boa/src/builtins/string/mod.rs#L301

We should match on the value (or if let) and if its a regular expression throw a type error:

ctx.throw_type_error("First argument to String.prototype.startsWith must not be a regular expression")?;

There should be a test also

Same here: https://github.com/boa-dev/boa/blob/master/boa/src/builtins/string/mod.rs#L344
and here: https://github.com/boa-dev/boa/blob/master/boa/src/builtins/string/mod.rs#L390

Example JS

const str1 = 'Saturday night plans';
console.log(str1.startsWith(/saturday/));

Contributing: https://github.com/boa-dev/boa/blob/master/CONTRIBUTING.md
Debugging: https://github.com/boa-dev/boa/blob/master/docs/debugging.md

[pt2121]

Can I take a stab at this?