Use Cli::try_parse_from in cast docs snippets validation

[franciszekjob]

Which components does the task require to be changed? (think hard pls)

sncast

Description

Currently, sncast docs snippets are validated only using runner. Before running the command, we should check if it's at least valid using try_parse_from on Cli. ATM, we cannot do this, because Cli cannot be imported from main.rs.

In the scope of this issue, we should move Cli from main.rs to lib.rs (and other needed structs/functions) and use Cli::try_parse_from before using runner.