refactor: clean up unused code, fix typos, improve docs (#7)

* refactor: clean up unused code, fix typos, improve docs

- Remove unused 'killed' const and 'unitTypes' var (staticcheck U1000)
- Replace regexp with strings.TrimSuffix+switch in isFailed for consistency
- Fix typo: 'programatically' -> 'programmatically'
- Fix typo: 'an an int' -> 'as an int' in README and helpers.go
- Add missing godoc comments on exported helper functions
- Bump minimum Go version from 1.18 to 1.21

* refactor: use unused constants instead of removing them

- Export unitTypes as UnitTypes and add HasValidUnitSuffix helper
- Use killed const (exit code 130) in execute() to detect SIGINT
- Update go.mod to go 1.26

structs.go

@@ -1,5 +1,7 @@
 package systemctl
 
+import "strings"
+
 type Options struct {
 	UserMode bool
 }
@@ -12,7 +14,8 @@ type Unit struct {
 	Description string
 }
 
-var unitTypes = []string{
+// UnitTypes contains all valid systemd unit type suffixes.
+var UnitTypes = []string{
 	"automount",
 	"device",
 	"mount",
@@ -26,3 +29,14 @@ var unitTypes = []string{
 	"target",
 	"timer",
 }
+
+// HasValidUnitSuffix checks whether the given unit name ends with a valid
+// systemd unit type suffix (e.g. ".service", ".timer").
+func HasValidUnitSuffix(unit string) bool {
+	for _, t := range UnitTypes {
+		if strings.HasSuffix(unit, "."+t) {
+			return true
+		}
+	}
+	return false
+}