This adds best-effort only support to the visualisers for handling
stand-alone `Var` nodes.  Most of the changes are actually in `qasm3`,
since the visualisers use internal details of that to handle the nodes.

This commit decouples the visualisers _slightly_ more from the inner
workings of the OQ3 exporter by having them manage their own
variable-naming contexts and using the encapsulated `_ExprBuilder`,
rather than poking into random internals of the full circuit exporter.
This is necessary to allow the OQ3 exporter to expand to support these
variables itself, and also for the visualisers, since variables may now
be introduced in inner scopes.

This commit does not attempt to solve many of the known problems around
zero-operand "gates", of which `Store` is one, just leaving it un-drawn.
Printing to OpenQASM 3 is possibly a better visualisation strategy for
large dynamic circuits for the time being.

Co-authored-by: Matthew Treinish <[email protected]>