Emit warnings in #[function] macro expansion when invalid arguments used

[hiltontj]

This is a follow on for #69 to cover the one TODO that was not cleaned up by #72.

The TODO can be found here:

serde_json_path/serde_json_path_macros/src/internal/func/args.rs

Lines 10 to 30 in 5dd145d

	impl Parse for FunctionMacroArgs {
	fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
	let mut args = Self::default();
	while !input.is_empty() {
	let lookahead = input.lookahead1();
	if lookahead.peek(kw::name) {
	if args.name.is_some() {
	return Err(input.error("expected only a single `name` argument"));
	}
	let name = input.parse::<StrArg<kw::name>>()?.value;
	args.name = Some(name);
	} else {
	// TODO - may want to warn here when found a invalid arg - see how
	// tracing::instrument stores warnings and emits them later when generating the
	// expanded token stream.
	let _ = input.parse::<proc_macro2::TokenTree>();
	}
	}
	Ok(args)
	}
	}

I believe that if unsupported arguments are passed to the #[function] macro, it will just ignore them, but should instead emit compiler warnings.