Moved StepDefinition to the formatters pkg

[lonnblad]

This change would finish the move of all the needed extra exports for custom formatters to one pkg and then concentrate the exports in the base pkg to the ones needed for using godog to execute tests.

This would mean that the base of a custom formatter would look like this:

package customformatter

import (
	"io"

	"github.com/cucumber/godog"
	"github.com/cucumber/godog/formatters"
	"github.com/cucumber/messages-go/v10"
)

func init() {
	formatters.Format("custom", "Custom formatter", customFormatterFunc)
}

func customFormatterFunc(suite string, out io.Writer) formatters.Formatter {
	return &customFmt{suiteName: suite, out: out}
}

type customFmt struct {
	suiteName string
	out       io.Writer
}

func (f *customFmt) TestRunStarted()                                                        {}
func (f *customFmt) Feature(*messages.GherkinDocument, string, []byte)                      {}
func (f *customFmt) Pickle(*godog.Scenario)                                                 {}
func (f *customFmt) Defined(*godog.Scenario, *godog.Step, *formatters.StepDefinition)       {}
func (f *customFmt) Passed(*godog.Scenario, *godog.Step, *formatters.StepDefinition)        {}
func (f *customFmt) Skipped(*godog.Scenario, *godog.Step, *formatters.StepDefinition)       {}
func (f *customFmt) Undefined(*godog.Scenario, *godog.Step, *formatters.StepDefinition)     {}
func (f *customFmt) Failed(*godog.Scenario, *godog.Step, *formatters.StepDefinition, error) {}
func (f *customFmt) Pending(*godog.Scenario, *godog.Step, *formatters.StepDefinition)       {}
func (f *customFmt) Summary()                                                               {}

[codecov]

Codecov Report

Merging #332 into master will increase coverage by 0.08%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   80.04%   80.13%   +0.08%     
==========================================
  Files          23       23              
  Lines        2230     2260      +30     
==========================================
+ Hits         1785     1811      +26     
- Misses        342      346       +4     
  Partials      103      103              

Impacted Files	Coverage Δ
formatters/fmt.go	100.00% <ø> (ø)
internal/models/stepdef.go	58.92% <0.00%> (-2.19%)	⬇️
internal/formatters/fmt_base.go	86.33% <100.00%> (ø)
internal/formatters/fmt_events.go	97.48% <100.00%> (+0.01%)	⬆️
internal/formatters/fmt_pretty.go	81.22% <100.00%> (ø)
internal/formatters/fmt_progress.go	93.90% <100.00%> (ø)
internal/storage/storage.go	91.42% <100.00%> (+0.98%)	⬆️
suite.go	90.72% <100.00%> (+0.31%)	⬆️
test_context.go	63.26% <100.00%> (+1.56%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce8036f...2fa3b9f. Read the comment docs.