Allow custom from_word and from_none behaviors in derived FromMeta impls #1994

Job	Run time
Lint with clippy	14s
Verify code formatting	17s
Test Rust nightly on ubuntu-latest	22s
Test Rust nightly on macos-latest	36s
Test Rust nightly on windows-latest	1m 0s
Test nightly with diagnostics feature	24s
Test Rust stable on ubuntu-latest	33s
Test Rust stable on macos-latest	27s
Test Rust stable on windows-latest	3m 11s
Test Rust 1.56.0 on ubuntu-latest	4m 16s
Test Rust 1.56.0 on windows-latest	5m 3s
	16m 23s

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