Add support for decimal types

### What is wrong?

We should provide a type to represent decimals.

Prior art:

- https://docs.soliditylang.org/en/develop/types.html#rational-and-integer-literals
- https://vyper.readthedocs.io/en/latest/types.html?highlight=decimal#decimals

The issue names no implementation files, tests, or entry points. Start by reviewing the linked Solidity and Vyper decimal-type references and determine the required scope and semantics; the issue does not define what completion or validation should include.

Written by the indexing model from the issue text.