Migrate from Flow to TypeScript

[lydell]

This PR gets rid of Flow and switches to TypeScript. This should make it easier to work on the code base, since TypeScript has much better tooling support. It also let us get rid of an annoying Flow+Prettier workaround for type aliases.

I went with JSDoc instead of .ts files, since it was easier, and preserves all workflows in the repo (the code stays valid standard JS) – even with Flow we used their comment syntax instead of their own type annotation syntax.

While this kind of task is pretty well suited to LLM:s, I made this PR without any usage of LLM:s. Some of the boring translations were done by this script I threw together:

import fs from 'node:fs';

function convert(code) {
  return code.replace(
    /((?:\/\/.*\n)*)^(async )?function (\w+)(?:\/\*:: <(\w+)> \*\/)?\(([^()]+)\)\s*(?:\/\*:([^*]+)\*\/\s*)?\{/gm,
    (
      _match,
      comment,
      async_ = '',
      name,
      generics,
      args,
      return_ = ' TODO '
    ) => {
      const types = args.split(/\*\/,\s*/).map((s) => {
        const [param, type = 'TODO'] = s.split(' /*: ');
        return `@param { ${type.trim().replace(/ \*\//, '')} } ${param.trim()}`;
      });
      const argsOnly = args.replace(/\/\*[^*]*\*\//g, '').trim();
      return `
/**${
        comment.length > 0
          ? '\n * ' +
            comment
              .split('\n')
              .map((line) => line.replace(/^\/\/\s*/, ''))
              .join('\n * ')
          : ''
      }${generics ? 'TODO generics: ' + generics : ''}
 * ${types.join('\n * ')}
 * @returns {${return_}}
 */
${async_}function ${name}(${argsOnly}) {
`.trim();
    }
  );
}

for (const fileName of fs.readdirSync('lib')) {
  const file = `lib/${fileName}`;
  const code = fs.readFileSync(file, 'utf8');
  const newCode = convert(code);
  fs.writeFileSync(file, newCode);
}