allows manifest file to be parameterized

[robacarp]

Purpose

fixes #1382 - manifest json file is hard coded to be laravel-mix specific.

Description

This patch allows src/app.cr in a generated application to be changed to this:

-# Load the asset manifest in public/mix-manifest.json
-Lucky::AssetHelpers.load_manifest
+Lucky::AssetHelpers.load_manifest "public/mix-manifest.json"

For folks using rollup, parcel, vite, or whatever hot javascript packaging engine suits them, this top level hook for replacing the manifest file is important.

Notably, this doesn't require the change to app.cr so this isn't a breaking change. It just provides an API where non existed before.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[stephendolan]

I love this! Thanks, @robacarp!

[jwoertink]

Nice! This is a great solution. Thanks for knocking this out ❤️