CVE-2021-37634: LeafKit allows XSS with untrusted user input

This affects anyone passing unsanitised data to Leaf’s variable tags. Before this fix, Leaf would not escape any strings passed to tags as variables. If an attacker managed to find a variable that was rendered with their unsanitised data, they could inject scripts into a generated Leaf page, which could enable XSS attacks if other mitigations such as a Content Security Policy were not enabled.