feat: add support for OpenSearch browser hinting.

[andypiper]

This PR aims to be a clean patch that adds support for telling web browsers about a Postmarks instance as a search option, via the (now dated, but still pretty well-supported) OpenSearch XML syntax.

• adds a small additional package dependency (xml2js)
• the Postmarks server now serves /opensearch.xml via a new route handler
• added <link> hint pointing to the search provider document to the main.hbs template so it is included in every page

Works in Firefox, MS Edge, Chrome, Arc; no sign of this feature in Vivaldi or Opera.

May or may not be widely useful, but it is also (sort of) another half of the "send to" story, which I've been lightly tinkering with; there may also be an option to build this kind of search engine hinting into a browser extension.

[ckolderup]

this is so cool! I guess I always knew there was a standard to make this work in Firefox, but I've never actually looked up what it was.

Tested on my test Glitch app and it works with Firefox as expected. Thanks!