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.

thirdparty: vendor tomli The next commit will introduce a .toml file to abstract configitems away from Python. Python 3.11 has a toml read-only library (`tomllib`), which gives us a way out of vendoring eventually. For now, we vendor the backport, specifically version 1.2.3 which is still compatible with Python 3.6.

configitems: add `documentation` field It may be useful to expose documentation information in the help in some form. This will be populated in a future changeset by using the current comments that are relevant for users.

configitems: use standard "dynamicdefault" approach in edge case This makes for fewer edge cases, which will help a future patch.