Allow printing of @enum values to TOML as Strings

[johnomotani]

Adds a method of TOML.Internals.Printer.to_toml_value that handles Enum values, allowing Enums to be printed to TOML format strings or files.

Counterpart to the get(d::Dict, key, default::Enum) that lets us read Enums from Dicts, which means we can read them from TOML files by supplying a value of the Enum as the default.

Slightly hacky because TOML.Internals.Printer.to_toml_value appears to be an internal method, not part of the public API, but TOML.jl does not change very much, so in practice this should be fine.

[johnomotani]

On second thoughts, this is a bad solution. Modifying TOML.Internals.Printer.to_toml_value() means that we would change the behaviour of TOML.print() (i.e. how it handles Enum) as a side effect whenever moment_kinetics is loaded. I think a better solution is to have our own function for writing to TOML - see upcoming PR.