Update substring filter to match spec

[simon-kolkmann]

This PR closes #4

[jsumners]

Thank you for taking this on. It is really appreciated.

My initial feedback won't really work as line comments, so I'm going to do it in this top-level post.

We are going to need to deprecate the subX fields in the same manner as:

filter/lib/index.js

Lines 24 to 30 in e70a7cb

	/**
	* @deprecated 2022-06-26 Use `parseString` instead.
	*/
	parse: (string) => {
	deprecations.emit('LDAP_FILTER_DEP_001')
	return parseString(string)
	},

That's not the best example to follow because it isn't tested. So take a look at the following links for what we should really do:

• https://github.com/ldapjs/messages/blob/6ce6e35dee7a188d8154537831f2f5c3b59493af/lib/ldap-message.js#L26-L30
• https://github.com/ldapjs/messages/blob/6ce6e35dee7a188d8154537831f2f5c3b59493af/lib/ldap-message.test.js#L29-L56
• https://github.com/ldapjs/messages/blob/6ce6e35dee7a188d8154537831f2f5c3b59493af/lib/messages/abandon-request.js#L42-L50
• https://github.com/ldapjs/messages/blob/6ce6e35dee7a188d8154537831f2f5c3b59493af/lib/messages/abandon-request.test.js#L46-L67

After factoring in that pattern to this change, we'll review any other items.

[simon-kolkmann]

Thanks for your feedback! I have added and tested the deprecation messages.

[jsumners]

Looks good to me.

Thank you. Thankfully we won't need to keep this patch around too long since Node.js 16 is EOL this September.