feat: experiments logging improvements (#2049)

* feat: warn when enabling inactive experiments

* feat: TASK_ environment prefix

* feat: calculate experiment enabled/active instead of storing

* refactor: rename GetTaskVar to GetTaskEnv

* feat: experiments tests

internal/experiments/experiment_test.go

@@ -0,0 +1,74 @@
+package experiments_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/go-task/task/v3/internal/experiments"
+)
+
+func TestNew(t *testing.T) {
+	const (
+		exampleExperiment    = "EXAMPLE"
+		exampleExperimentEnv = "TASK_X_EXAMPLE"
+	)
+	tests := []struct {
+		name          string
+		allowedValues []string
+		value         string
+		wantEnabled   bool
+		wantActive    bool
+		wantValid     error
+	}{
+		{
+			name:        `[] allowed, value=""`,
+			wantEnabled: false,
+			wantActive:  false,
+		},
+		{
+			name:        `[] allowed, value="1"`,
+			value:       "1",
+			wantEnabled: false,
+			wantActive:  false,
+			wantValid: &experiments.InactiveError{
+				Name: exampleExperiment,
+			},
+		},
+		{
+			name:          `[1] allowed, value=""`,
+			allowedValues: []string{"1"},
+			wantEnabled:   false,
+			wantActive:    true,
+		},
+		{
+			name:          `[1] allowed, value="1"`,
+			allowedValues: []string{"1"},
+			value:         "1",
+			wantEnabled:   true,
+			wantActive:    true,
+		},
+		{
+			name:          `[1] allowed, value="2"`,
+			allowedValues: []string{"1"},
+			value:         "2",
+			wantEnabled:   false,
+			wantActive:    true,
+			wantValid: &experiments.InvalidValueError{
+				Name:          exampleExperiment,
+				AllowedValues: []string{"1"},
+				Value:         "2",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Setenv(exampleExperimentEnv, tt.value)
+			x := experiments.New(exampleExperiment, tt.allowedValues...)
+			assert.Equal(t, exampleExperiment, x.Name)
+			assert.Equal(t, tt.wantEnabled, x.Enabled())
+			assert.Equal(t, tt.wantActive, x.Active())
+			assert.Equal(t, tt.wantValid, x.Valid())
+		})
+	}
+}