std/os: add the Getenv function

std/os/proc.jule

@@ -47,4 +47,12 @@ fn Chdir(path: str)! {
 	if !ok {
 		error(getLastOsError())
 	}
+}
+
+// Retrieves the value of the environment variable named by the key.
+// It returns the value, which will be empty if the variable is not present.
+// To distinguish between an empty value and an unset value, use [LookupEnv].
+fn Getenv(key: str): str {
+	v, _ := LookupEnv(key)
+	ret v
 }