docs: list all supported languages for --lang

[Socialpranker]

Disclosure: this patch was prepared with AI assistance (Claude Opus 5). I checked the enum and the cargo features myself before opening this.

Problem

The --lang entry in docs/src/usage/configuration.md says:

Currently, this option can be set to either kotlin, swift, go, or typescript.

AvailableLanguage in cli/src/args.rs has six variants:

pub enum AvailableLanguage {
    Kotlin,
    Scala,
    Swift,
    Typescript,
    #[cfg(feature = "go")]
    Go,
    #[cfg(feature = "python")]
    Python,
}

So scala was missing entirely, and python was missing as well.

Scala is not an obscure omission — docs/src/usage/usage.md lists it among the supported languages and uses it in an example (typeshare ./my_rust_project --lang=scala ...), and the README mentions it too. The gap is specific to this one file.

Fix

List all six, and note the feature gating for the two that have it ([features] go = [], python = [] in cli/Cargo.toml). The phrasing follows the --go-package entry a few lines further down in the same file, which already describes its feature requirement that way.