feat: add WithClock option for custom time source injection

[CybotTM]

Summary

• Add Clock type (func() time.Time) for custom time functions
• Add WithClock Option to inject custom time source into Cron
• Update now() method to use custom clock when configured
• Add comprehensive tests for clock functionality

Motivation

Addresses #4 - Custom time source injection

Testing cron scheduling logic often requires control over time. The existing implementation always uses time.Now(), making tests either slow (waiting for real time) or requiring complex mocking.

This feature allows injecting a custom time source, enabling:

• Deterministic tests with fixed timestamps
• Fast-forwarding time in tests
• Simulating specific dates/times for edge cases

Usage

// Fixed time for testing
fixedTime := time.Date(2024, 1, 1, 12, 0, 0, 0, time.UTC)
c := cron.New(cron.WithClock(func() time.Time { return fixedTime }))

// Or with controllable time
var currentTime atomic.Value
currentTime.Store(time.Now())
c := cron.New(cron.WithClock(func() time.Time { return currentTime.Load().(time.Time) }))

Test plan

• TestWithClock - verify clock is set and used
• TestWithClockAndLocation - verify clock respects location setting
• TestWithClockNilFallback - verify default behavior when no clock set
• All existing tests pass
• Race detection passes
• Linter passes

Closes #4