JavaScript source maps contain incorrect URL

[valtlai]

Source maps generated by the Terser plugin have an incorrect URL 0 in the sources array.

A test case:

• _config.js:

  import lume from "lume/mod.js";
  import terser from "lume/plugins/terser.js";
  
  const site = lume()
    .use(terser({ sourceMap: true }));
  
  export default site;

• index.njk:

  <script src="/foo.js"></script>

• foo.js:

  console.log("Test!");

That results the following source map:

• foo.js.map:

  {"version":3,"sources":["0"],"names":["console","log"],"mappings":"AAAAA,QAAQC,IAAI","file":"/foo.js"}

  Should have foo.js instead of 0.

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Edit: The issue originally covered the PostCSS plugin too, but I think its source maps are correct after all.