[Feature Suggestion] Ability to load banner and footer from files

Sometimes banner and footer strings can mess the command line up, but allow loading from files can circumvent this.

I suggest adding two new arguments, `--prepend:` and `--append:`.

Start by locating esbuild's command-line argument handling and the existing banner and footer string handling. Check how file inputs are represented and define completion as supporting --prepend: and --append: so banner and footer content can be loaded without putting long strings directly on the command line.

Written by the indexing model from the issue text.