utils: replace isPlainObject with a robuster implementation

[ocavue]

Copy the code from the NPM package is-plain-obj to replace our current isPlainObject implementation. Closes #21.

I choose to copy code instead of adding it as a dependency because of the following reasons:

1. is-plain-obj is an ESM-only package, while noble is an ESM-CJS dual package.
2. @noble/ciphers is designed as zero-dependency.

[maccman]

@paulmillr Just bumping this one. I suspect you'll want to merge it.

[paulmillr]

Solved in 2e49a16. It's only used internally, so it was simplified.

[paulmillr]

0.5.0 is out