toml: implement decode method for Doc

[ttytm]

I.e. toml.parse_file returns a Doc. So it probably makes sense to implement a decode method for it to simplify the decoding process.

It's includes #19317, which is a separate change, I would rebase it after it is resolved.

🤖 Generated by Copilot at dacfe18

This pull request adds a new feature and tests to the toml module. The feature allows decoding TOML documents into custom structs with compile-time type checking. The tests cover the new feature and the error handling of the decode function.

🤖 Generated by Copilot at dacfe18

• Add a compile-time check to decode function of toml module to ensure struct type argument (link)
• Add a decode method to Doc struct to allow decoding a parsed TOML document into a custom struct type (link)
• Add two test functions to vlib/toml/tests/encode_and_decode_test.v to test the new decode method of Doc struct and the error handling of decode function of toml module (link)
• Add an enum type Title and a struct type Employee to vlib/toml/tests/encode_and_decode_test.v to use as target types for decoding (link)

[ttytm]

Rebased and included stronger tests as suggested.