Add support for Either values

[schickling]

Is your feature request related to a problem? Please describe.

Currently schema doesn't support Either values but should.

Describe the solution you'd like

Add support for S.either.

Describe alternatives you've considered

Additional context

[patroza]

Started preparing Either in https://github.com/Effect-TS/schema/pull/35 https://github.com/Effect-TS/schema/pull/35/commits/2183f28eb36b05b10fc1e19f6c72bdc770b01a7f