feat: add JamTime, TimeSlot and Epoch implementation

[aranw]

First go at the JamTime stuff. Still got some testing to do for this package

I renamed the package from time to jamtime so that it did not clash with the standard library time package. Without remaining we'd of had to alias one of the packages

import (
    "time"
    jamtime "github.com/eigerco/strawberry/internal/time"
)

or

import (
    stdtime "time"
    "github.com/eigerco/strawberry/internal/time"
)

[bamzedev]

I think we should generally return errors.