feature(cli): allow multiple template sources on command line

[jkylling]

Enable passing multiple template files/URLs to limactl create and limactl start.

Example usage:
limactl create template:docker my-overrides.yaml
limactl create https://example.com/base.yaml secrets.yaml

This is equivalent to using the base: property in YAML:
base: [https://example.com/template.yaml, $PWD/secrets.yaml]

Fixes #3404

[jandubois]

This PR drops most of the special cases for specifying templates on the command line. Even the simple single-arg case no longer works:

❯ limactl start template:alpine
FATA[0000] cannot derive instance name from "template:alpine": filename "template:alpine" is invalid: identifier "template:alpine" must match ^[A-Za-z0-9]+(?:[._-](?:[A-Za-z0-9]+))*$

Using an image URL no longer creates a short instance name:

❯ limactl start https://dl-cdn.alpinelinux.org/alpine/v3.23/releases/cloud/nocloud_alpine-3.23.2-aarch64-uefi-cloudinit-r0.qcow2
? Creating an instance "nocloud_alpine-3.23.2-aarch64-uefi-cloudinit-r0.qcow2" Proceed with the current configuration
...

It looks like the special handling for - (stdin) only works for the first argument.

I haven't reviewed the code yet, but we should probably start by creating a bunch of BATS tests for the current behaviour, so we can be sure refactoring to implement multiple input templates is not breaking any of them.