embed, cmd/go: add a way to exclude a file from embedding

Problem: if you embed a directory, it may embed unwanted files from the directory.

Proposal: Comments with `//go:embed-exclude` apply a filter of files to exclude from the embed.

Example:

Given a directory with `static/a.txt` and `static/.dot`:

```
//go:embed static
//go:embed-exclude .*
var staticFiles embed.FS
```

will just embed `static/a.txt`.

Refs. #42321.