Add mutable configurable

[aidantwoods]

Configurables are designed to be immutable because it makes working with them and reasoning about their values much easier (see: #708 (comment)), however in some very special cases mutability might be required to create special behaviours (envisioned use case for this is mostly where definitions are recorded and need to be accessible by the whole parsing tree).

To quote the embedded description for the MutableConfigurable interface:

Beware that the values of MutableConfigurables are NOT stable. Values SHOULD
be accessed as close to use as possible. Parsing operations sharing the same
State SHOULD NOT be triggered between where values are read and where they
need to be relied upon.