Provide config schema

[kije]

Provide a config schema similar to what Starship does:

(from https://starship.rs/config/#configuration)

# Get editor completions based on the config schema
"$schema" = 'https://starship.rs/config-schema.json'

# ...

[stephenleo]

Shipped in #197. A JSON schema is now published at https://cship.dev/config-schema.json, generated from cship's config structs so it stays in sync (CI fails if it drifts).

Add this to the top of your cship.toml or starship.toml to get editor autocomplete/validation for the [cship] section (e.g. with VS Code's Even Better TOML):

"$schema" = 'https://cship.dev/config-schema.json'

You can also generate it locally with cship config-schema. Docs: https://cship.dev/configuration#editor-schema