Introduce the 'OS' enum and improve error messages

[chshersh]

Currently, tool-sync matches the value of env::consts::OS with the hardcoded string to extract the relevant part of the asset name:

https://github.com/chshersh/tool-sync/blob/df7d4768c9cff2a91616bd9686bc928a52fc8c10/src/model/asset_name.rs#L18-L22

I propose to introduce an enum with 3 constructors. This should allow us to support more OSes in the future if we ever need. And it'll help to improve error messages easily.

So the issue includes the following tasks:

• [x] Create a new enum with 3 OSes
• [x] Create a function to get the current value of enum based on the above matching logic
• [ ] Use enum in get_name_by_os
• [ ] Implement the Display trait for enum to match the TOML configuration format
• [ ] Improve the error message below

https://github.com/chshersh/tool-sync/blob/df7d4768c9cff2a91616bd9686bc928a52fc8c10/src/model/release.rs#L36-L40

It should be something like:

"Unknown asset selector for the current OS. Specify 'asset_name.{}' in the config.",
get_os(),