This addresses sindresorhus#142 by creating a new macro that can be used inside
`@Observable` classes.

The macro is implemented in a new `DefaultsMacros` module. The
decision to do so is based on the introduction of a new dependency on
`swift-syntax`, and to keep the main module dependency-free.

Tests for the macro declaration were in the wrong test target and the
test method names were not accurate.

Added another test target to actually test that the macro works when
used in an @observable class.

This is to be consistent with the `@Default` property wrapper.

In the macro tests, I had to move the @observable classes out of the
test because linter was asking to mark them as private but doing so
was causing the @observable macro to error.

We cannot overload @default, otherwise it is impossible to use the
property wrapper version in the same source file as the macro as the
later takes precedence.