Co-authored-by: jerry.wang <jerry.wang>
Co-authored-by: John Roesler <johnrroesler@gmail.com>

We should use `(*regexp.Regexp).MatchString` instead of
`(*regexp.Regexp).Match([]byte(...))` when matching string to avoid
unnecessary `[]byte` conversions and reduce allocations.

Example benchmark:

func BenchmarkMatch(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := timeWithSeconds.Match([]byte("06:18:01")); !match {
			b.Fail()
		}
	}
}

func BenchmarkMatchString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := timeWithSeconds.MatchString("06:18:01"); !match {
			b.Fail()
		}
	}
}

goos: linux
goarch: amd64
pkg: github.com/go-co-op/gocron
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 7213512	       223.8 ns/op	       8 B/op	       1 allocs/op
BenchmarkMatchString-16    	 8484169	       145.8 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/go-co-op/gocron	4.114s

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

…immediate error #600 (#603)

* fix(scheduler): ensure negative intervals given to `Every` return an immediate error #600

* Update Every func doc

* fix error check in tests

Co-authored-by: John Roesler <johnrroesler@gmail.com>

* Apply suggestions from code review

* Apply suggestions from code review

* Update scheduler_test.go

---------

Co-authored-by: John Roesler <johnrroesler@gmail.com>