rhg: use default configitem in `cat` Now that we've declared the items, we can get rid of the manual fallback.

rhg: add config defaults to configitems.toml This will allow us to use the same default config mechanic as the rest of the code.

rust-blackbox: use `is_extension_enabled` config helper It's there, may as well use it to make the code clearer and less bug-prone.

configitems: move blackbox's config items to the new configitems.toml In order for the Rust code to gain access to default values of in-core extensions that have a Rust implementation, we need to centralize them alongside the core items declarations. This is the first and so far only one of the extensions that have gained Rust support, I don't think it's worth the churn to move the rest of the extension's configitems yet.

rust-config: add config getters that don't fall back to defaults This is useful in cases where we access config items that are more... lenient with their types than a fresh new system would allow. For now there is only a single use of this, but we might get more later.

rust-config: add support for default config items Now that configitems.toml exists, we can read from it the default values for all core config items. We will add the devel-warning for use of undeclared config items in a later patch when we're done adding the missing entries for `rhg`.

configitems: declare items in a TOML file Mercurial ships with Rust code that also needs to read from the config. Having a way of presenting `configitems` to both Python and Rust is needed to prevent duplication, drift, and have the appropriate devel warnings. Abstracting away from Python means choosing a config format. No single format is perfect, and I have yet to come across a developer that doesn't hate all of them in some way. Since we have a strict no-dependencies policy for Mercurial, we either need to use whatever comes with Python, vendor a library, or implement a custom format ourselves. Python stdlib means using JSON, which doesn't support comments and isn't great for humans, or `configparser` which is an obscure, untyped format that nobody uses and doesn't have a commonplace Rust parser. Implementing a custom format is error-prone, tedious and subject to the same issues as picking an existing format. Vendoring opens us to the vast array of common config formats. The ones being picked for most modern software are YAML and TOML. YAML is older and common in the Python community, but TOML is much simpler and less error-prone. I would much rather be responsible for the <1000 lines of `tomli`, on top of TOML being the choice of the Rust community, with robust crates for reading it. The structure of `configitems.toml` is explained inline.