Adds an alternative TOML configuration format to Lake.
* Uses TOML v1.0.0 and is fully specification compliant (tested via
[toml-test v1.4.0](https://github.com/toml-lang/toml-test/tree/v1.4.0)).
* Supports package configuration options, Lean libraries, Lean
executables, and dependencies.
* TOML configurations can be generated for new projects via `lake
new|init <pkg> <template>.toml`.
* Supported configurations can be converted to/from TOML via `lake
translate-config <lang>`.
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