Support single-dimension image resizing

[Charlie284]

Summary

• add with_width and with_height image asset options
• preserve the source aspect ratio when the CLI resizes a single dimension
• cover width- and height-constrained output dimensions with focused tests

Closes #5458

Testing

• cargo fmt --all -- --check
• cargo test -p dioxus-cli opt::image::tests
• cargo check -p dioxus-cli
• cargo test -p manganis-core
• cargo clippy -p dioxus-cli --tests -- -D warnings
• cargo test --workspace --tests

[nicoburns]

Maybe if you call both .with_width and .with_height it ought to apply Manual?