Support `Decimal` serialization?

[goodboy]

Love this lib btw, it's been a huge headache trying to write a certain style of TOML with anything else; code base is also very easy to nav, so big 👍🏼

So the tomli lib has this and it'd sure be handy to have here instead of having to traverse a tree to convert everything to str first 😂

I think to start it'd be good enough to just serialize to String since presumably on parse the client code should know if it wants to cast to decimal (and obvsly TOML doesn't spec a decimal type); though i guess it wouldn't be too hard to add some logic + flag to .parser.Parser._parse_number() ?

[goodboy]

As per discussion in #294 it makes more sense to just expect users to define a custom encoder since there's no type support in the TOML spec for Decimal:
#294 (comment)

Those looking for a solution can defer to @frostming's comment linked above.