refactor: use SafeMap rather than plain object hash for internal results

[shadowspawn]

This came out of discussion around potential for prototype pollution. Use a Map rather than Object internally. Convert to Object for returning to client.

See #32 (comment)

Take two, previous draft in #57

ToDo: add some related tests from #57 (review)

[bcoe]

Looks great, let's just add some tests for prototype pollution and keys named for methods on the prototype.

https://github.com/yargs/yargs-parser/blob/main/test/yargs-parser.cjs#L3712
https://github.com/yargs/yargs-parser/blob/main/test/yargs-parser.cjs#L1011

[shadowspawn]

I am getting pretty used to the primordial routines now, and this got broken by other PR again. Adding some prototype pollution tests is probably still a good idea independent of SafeMap. I won't keep this open as a placeholder for further action though.