fix: support @layer rule

[siriwatknp]

Issue

The stylis-plugin-rtl does not work when a rule has @layer as a parent.

To reproduce, add this test to the existing codebase at src/stylis-plugin-rtl.test.ts:

it("works for layer rules", () => {
    expect(
      stylis(`
        @layer default {
          .cls {
            margin-right: 32px;
            & .first-child {
              margin-right: 32px;
            }
          }
        }
      `)
    ).toMatchInlineSnapshot(
      `"@layer default{.cls{margin-left:32px;}.cls .first-child{margin-left:32px;}}"`
    );
  });

The test will fail.

Dependencies

@layer is introduced in stylis after v4.0, so need to update the dependencies and types.