Add a way to prevent templates impacting remaining content

[kezz]

Something like <my_template/> which would resolve the template whilst preventing any tags inserted from bleeding into remaining strings.

Essentially shorthand for <span><my_template></span>, considering KyoriPowered/adventure-text-minimessage#178.

[zml2008]

This should be easy with #672 -- it just depends on the value of Tag#allowsChildren(). We could incorporate the same logic into the parser in order to support / for self-closing tags.