allow scheduler functions pertaining to a job to be called in any order

[JohnRoesler]

What does this do?

Previous to this change, odd behavior would occur when calling a method like .Tag() prior to .Every() -> like in #314

See the description on the new scheduler field

// tracks whether we're in a chain of scheduling methods for a job
// a chain is started with any of the scheduler methods that operate
// upon a job and are ended with one of [ Do(), Update() ] - note that
// Update() calls Do(), so really they all end with Do().
// This allows the caller to begin with any job related scheduler method
// and only with one of [ Every(), EveryRandom(), Cron(), CronWithSeconds(), MonthFirstWeekday() ]
inScheduleChain bool

and look what you can do now (from tests)

_, err := s.Tag("1").SingletonMode().Milliseconds().EveryRandom(100, 200).Do(func1)
require.NoError(t, err)

_, err = s.Monday().Every(4).Tag("2").Do(func2)
require.NoError(t, err)

_, err = s.Months(1).Tag("3").Every(1).SingletonMode().At("1:00").Do(func3)
require.NoError(t, err)

Which issue(s) does this PR fix/relate to?

related #314

List any changes that modify/break current functionality

Have you included tests for your changes?

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes