Support banner in Bun.build

[synthlace]

What is the problem this feature would solve?

I need functionality that allows me to prepend text to a bundled file.
This text is often called a banner, the minify option should ignore this text as it may contain comments.

In my case, a banner is metablock that is required by userscript.

Current workaround

await Bun.build({
  entrypoints: ["./src/index.tsx"],
  outdir: "./dist",
  minify: true,
});

const metablock = `\
// ==UserScript==
// @key value
// ==/UserScript==
\n`;

await appendMetablock("./dist/index.js", metablock);

async function appendMetablock(path: string, metablock: string) {
  const file = Bun.file(path);
  const fileText = await file.text();

  let newContent = metablock + fileText;
  await Bun.write(path, newContent);
}

TODO comment:

bun/src/bundler/bundle_v2.zig

Line 7743 in 39b1c01

// TODO: banner

Similar issue: #6119

Originated from discord help channel

What is the feature you are proposing to solve the problem?

New argument for Bun.build named banner or preamble (ref terser).
Passed text should be ignored by minify parameter and affect only entrypoints.

What alternatives have you considered?

No response

[7heMech]

Fixed by #14370