Support free bindings for `t-call`

[Desdaemon]

When using t-call, a lot of bindings are assumed to be either from the template context, or supplied by callees. However, as a callee, it is impossible to know and remember all the possible bindings that have special meaning in the original templates.

A solution would be to collect all free bindings, i.e. identifiers used without a matching t-as or t-set definition, and allow completion/jumpdefs/references/etc.