`unseparated_literal_suffix`

This commit adds a configuration `literal-suffix-style` to enforce a
specific style for unseparated_literal_suffix. The configuration accepts
two values:
- "separated"
    enforce all literals to be written separately (e.g. `123_i32`)
- "unseparated"
    enforce all literals to be written as unseparated (e.g. `123i32`)

Not specifying a value means that there is no preference on style and
any style should not be warned.