Switch arg parsing to clap-derive

[tranzystorekk]

Switches arg parsing from vanilla clap to clap-derive, which reduces boilerplate code (e.g. manual parsing of arg strings to useful types like port numbers, automatic handling of default values)

Major changes:

• The site dir becomes a positional argument for all subcommands, instead of a global flag, so the usage becomes e.g. firn new my_dir
• Clap automatically parses arguments to their target types, rejecting invalid values with an error message (i.e. port number)
• The default for site dir is now ., instead of explicitly calling env::current_dir(). This shifts the interpretation of cwd to the shell and filesystem and should be fine in general, but let me know if that's not okay

Minor changes:

• The verbosity level is now u8, for logical coherence
• The "about" strings have been made more uniform and concise

[tranzystorekk]

The version, about and author parts of help output can also be automated to the values from the Cargo.toml, if it is desired, but I haven't enabled it yet

[teesloane]

I'm open to this change, it's nice. Some stuff is still broken, however:

• the ./ prefix from #[clap(default_value = ".")] breaks the paths when running firn new | build | serve

If you can get those working (and please make sure to test locally to make sure they do work, since there are no tests yet), then I'll be happy to merge this.

[tranzystorekk]

Seems the glob crate can't handle the . in its patterns, I think the easiest solution is to revert to env::current_dir() after all 😅

EDIT: that also means that running sth like firn serve . never worked, would probably require canonicalizing the path.

[teesloane]

Nice - looks good to me. Thanks for contributing.