Move to clap_derive so CLI arguments are typed

[AndreasBackx]

Move to clap_derive so CLI arguments are typed

• Renames clap.rs to cli.rs because otherwise there's confusion between the clap crate and local module.
• Cli struct added that almost identically represents the current state of the CLI with no logical changes.

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--help Comparison

Before: https://gist.github.com/AndreasBackx/5945b366e989159f4669e7ba30c13239

After: https://gist.github.com/AndreasBackx/8929c8bde080eac0cafd33128210b0cc

Diff (ignoring whitespace changes due to table alignment):

1c1
< Screenshot tool for compositors implementing zwlr_screencopy_v1.
---
> Screenshot tool for wlroots based compositors implementing the zwlr_screencopy_v1 protocol.
9d8
<   -c, --cursor                      Enable cursor in screenshots
10a10
>   -c, --cursor                      Enable cursor in screenshots
12c12
<   -l, --listoutputs                 List all valid outputs
---
>   -l, --list-outputs                List all valid outputs
14c14
<       --chooseoutput                Present a fuzzy selector for outputs
---
>       --choose-output               Present a fuzzy selector for outputs
16a17
>

You can see that the only changes are:

• About is longer, this is now using the value from Cargo.toml instead of a duplicate text that was shorter.
• Some have a dash where the English words would have a space, e.g: "list-outputs" instead of "listoutputs". I've also made the old still work via an alias: https://gist.github.com/AndreasBackx/6025e91844e3d766d4264a01ae4d1a71

This seems like a tiny improvement? I plan to make further changes later, but I want to keep PRs separate.

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