feat: Implement SEARCH according to rfc5323

[susnux]

• Fixes Support for search method #205

This implements the WebDAV search request.
One real life example would be if you want to query recently changed files from Nextcloud, then this can be used like this:

const query = `<?xml version="1.0" encoding="UTF-8"?>
<d:searchrequest xmlns:d="DAV:" xmlns:ns="https://github.com/icewind1991/SearchDAV/ns">
	<d:basicsearch>
		<d:select>
			<d:prop>
			<d:getcontentlength />
			<d:getcontenttype />
			<d:getetag />
			<d:getlastmodified />
			</d:prop>
		</d:select>
		<d:from>
			<d:scope>
				<d:href>/files/YOUR_USER_ID/</d:href>
				<d:depth>infinity</d:depth>
			</d:scope>
		</d:from>
		<d:where>
			<d:gt>
				<d:prop>
					<d:getlastmodified/>
				</d:prop>
				<d:literal>${Math.round(Date.now() / 1000 - 24 * 60 * 60)}</d:literal>
			</d:gt>
		</d:where>
		<d:limit>
			<d:nresults>100</d:nresults>
			<ns:firstresult>0</ns:firstresult>
		</d:limit>
	</d:basicsearch>
</d:searchrequest>`;

// client with base path `SERVER/remote.php/dav`
client.search('/', { data: query })

[perry-mitchell]

This is amazing stuff @susnux! Honestly a huge feature. Thanks for all the work here.