layer() functions without arguments in @import are incorrectly treated as valid

[romainmenke]

Currently esbuild is failing this test: https://github.com/romainmenke/css-import-tests/tree/main/tests/002-sub-features/003-at-layer/018

@import url("green.css") layer; /* valid */
@import url("red.css") layer(); /* must be invalid */

It seems that esbuild treats layer() as valid and transpiles it to:

/* tests/002-sub-features/003-at-layer/018/red.css */
@layer {
  .box {
    background-color: red;
  }
}