Get rid if strings std library dependency in os (os std should never depend on another std library to avoid circular dependencies)

Author: monstermichl

std/os.tsh

@@ -1,9 +1,8 @@
-import "strings"
-
 func Shell() string {
 	shell := "unknown"
+	win := "%OS%"
 
-	if strings.HasPrefix("%OS%", "Win") {
+	if win[:3] == "Win" {
 		shell = "batch"
 	} else {
 		stdout, stderr, code := @ps("-p", "$$", "-o", "args=") | @cut("-d", " ", "-f1") | @grep("-o", "-e", `"[0-9a-zA-Z][0-9a-zA-Z]*$"`)