feat(shiny create): Support simpler syntax for --github flag

[gadenbuie]

This PR replaces the requirement for a full URL to a template hosted in a GitHub repo, replacing it with a shorter, more familiar syntax.

# In addition to the current incantation
shiny create -g https://github.com/posit-dev/py-shiny-templates/tree/main/map-distance

# These are now valid
shiny create --github posit-dev/py-shiny-templates/map-distance

shiny create --github posit-dev/py-shiny-templates/map-distance@main

Because there are many common ways to specify the ref and path portions of the GitHub spec, I ended up adding support for more than a few variations

• {repo_owner}/{repo_name}@{ref}:{path}
• {repo_owner}/{repo_name}:{path}@{ref}
• {repo_owner}/{repo_name}:{path}
• {repo_owner}/{repo_name}/{path}
• {repo_owner}/{repo_name}/{path}@{ref}
• {repo_owner}/{repo_name}/{path}?ref={ref}

Alternatively, the --template flag can now be combined with the --github flag, so these iterations are now the same as the above:

shiny create --github posit-dev/py-shiny-templates --template map-distance

shiny create --github posit-dev/py-shiny-templates@main --template map-distance

I'd really like to support listing available templates and asking the user to pick one, so that we could support

shiny create --github posit-dev/py-shiny-templates

with a missing --template flag. shiny create would download posit-dev/py-shiny-templates, find the available templates in the repo, and provide a menu to users.

While here, I've also improved the use of formatting and color in the CLI interface. Here's an example of the output seen when creating the map-distance template:

[cpsievert]

Are we sure this is always a Path? If so, why keep the str runtime check?

[gadenbuie]

Oops, I didn't mean to remove str from the input typing. I do think we could use

type=click.Path(dir_okay=True, file_okay=False)

for the --dir option. This does guarantee that dir is then a Path(). I'd still be on the fence about allowing str, wdyt?

[cpsievert]

Ok, yea, that seems sensible, and I don't see any harm in keeping the str type.

[gadenbuie]

I kept it but while looking at this realized we:

1. Should be asking for the destination directory earlier, e.g. before asking whether they want express/core syntax
2. Can better throw an error if the --dir is actually a file
3. Can improve the directory_prompt() experience by making dest_dir the first argument.

064f59f

[cpsievert]

LGTM once tests are passing