feat(commands/variable): add variable set command

Adds `variable set` command for creating GitLab environment variables.

Variable can be created for a project or a group specified with `--group | -g` flag.

Resolves #515

commands/variable/set/set_test.go

@@ -0,0 +1,43 @@
+package set
+
+import "testing"
+
+func Test_isValidKey(t *testing.T) {
+	type args struct {
+		key string
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			name: "key is empty",
+			args: args{
+				key: "",
+			},
+			want: false,
+		},
+		{
+			name: "key is valid",
+			args: args{
+				key: "abc123_",
+			},
+			want: true,
+		},
+		{
+			name: "key is invalid",
+			args: args{
+				key: "abc-123",
+			},
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isValidKey(tt.args.key); got != tt.want {
+				t.Errorf("isValidKey() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}