Prevent exception when passing directory instead of file

[jmorrell-cloudflare]

If you accidentally pass a directory path instead of a capnp config file to workerd serve $path you get an uncaught exception:

*** Uncaught exception ***
kj/filesystem-disk-unix.c++:365: failed: mmap: Invalid argument

This change adds an assertion that the provided path is a file type. Invoking workerd serve with a directory now results in the following error:

> workerd serve samples/tail-workers/
workerd serve: samples/tail-workers/: Config path is not a file.
workerd serve --help' for more information.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[jmorrell-cloudflare]

I have read the CLA Document and I hereby sign the CLA

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉

[kentonv]

Hmm, the code looks right to me but CI suggests it is not, somehow.

[danlapid]

Not sure I understand what's going on here, perhaps a rebase will solve it? 🤔