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

structs.go

@@ -11,18 +11,3 @@ type Unit struct {
 	Sub         string
 	Description string
 }
-
-var unitTypes = []string{
-	"automount",
-	"device",
-	"mount",
-	"path",
-	"scope",
-	"service",
-	"slice",
-	"snapshot",
-	"socket",
-	"swap",
-	"target",
-	"timer",
-}