Auto merge of #141272 - Shourya742:2025-05-18-modularize-config-module, r=Kobzol

modularize the config module bootstrap

Currently, our `config` module is quite large over 3,000 lines, and handles a wide range of responsibilities. This PR aims to break it down into smaller, more focused submodules to improve readability and maintainability:

* **`toml`**: Introduces a dedicated `toml` submodule within the `config` module. Its sole purpose is to define configuration-related structs along with their corresponding deserialization logic. It also contains the `parse_inner` method, which serves as the central function for extracting relevant information from the TOML structs and constructing the final configuration.

* **`rust`, `dist`, `install`, `llvm`, `build`, `gcc`, and others**: Each of these modules contains TOML subsections specific to their domain, along with the logic necessary to convert them into parts of the final configuration struct.

* **`config/mod.rs`**: Contains shared types and enums used across multiple TOML subsections.

* **`config/config.rs`**: Houses the logic that integrates all the TOML subsections into the complete configuration struct.

r? `@kobzol`

Author: bors