fix: match Deno version of prompt

[uncenter]

Removes an extra space in the prompt, adds the default message, and fixes the logic to match the Deno implementation.
I compared it to https://github.com/denoland/deno/blob/main/runtime/js/41_prompt.js#L33-L50, correct me if this is the wrong place please!

The main part of the Deno implementation looks like this:
https://github.com/denoland/deno/blob/a4ec7dfae01485290af91c62c1ce17a742dcb104/runtime/js/41_prompt.js#L34-L44

  defaultValue ??= null;


  if (!isatty(stdin.rid)) {
    return null;
  }


  if (defaultValue) {
    message += ` [${defaultValue}]`;
  }


  message += " ";

defaultValue is assigned to null if defaultValue is undefined or null (see nullish coalescing assignment (??=)). Then, if defaultValue is truthy, [${defaultValue}] is added to message.

I'll admit I'm quite confused why the defaultValue ??= null; line is needed; both null and undefined are not truthy soo the if (defaultValue) { part works the same either way...

Anyway, the logic for this shim implementation used to look like this:
https://github.com/denoland/node_shims/blob/main/packages/shim-prompts/src/index.ts#L35

defaultValue == null ? "" : `[${defaultValue}]`

Which, if expanded to an if statement and rearranged, would look like this:

if (defaultValue != null) {
  message +=  `[${defaultValue}]`;
}

Because the defaultValue ??= null; line is not needed (see above), the difference is defaultValue != null in the shim vs defaultValue in the Deno version. Hence, I've changed it to be accurate, just checking if defaultValue is truthy rather than if it equals null.

defaultValue ? ` [${defaultValue}]` : ""

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[uncenter]

I'm pretty sure we don't need this since it would default to undefined either way right?

[dsherret]

LGTM. Thanks!