Add `Context::style_mut` #7306

Job	Run time
android	40s
cargo-deny aarch64-apple-darwin	54s
cargo-deny aarch64-linux-android	56s
Check wasm32 + wasm-bindgen	3m 32s
Format + check + test	8m 24s
cargo-deny i686-pc-windows-gnu	49s
Check Windows	2m 12s
cargo-deny i686-pc-windows-msvc	58s
cargo-deny i686-unknown-linux-gnu	59s
cargo-deny wasm32-unknown-unknown	53s
cargo-deny x86_64-apple-darwin	47s
cargo-deny x86_64-pc-windows-gnu	54s
cargo-deny x86_64-pc-windows-msvc	58s
cargo-deny x86_64-unknown-linux-gnu	58s
cargo-deny x86_64-unknown-linux-musl	53s
cargo-deny x86_64-unknown-redox	51s
	25m 38s

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