Bump Rust Version and fix Plot Item Visibility in Demo

[bircni]

This pull request includes multiple changes to update the Rust toolchain version and improve code consistency and tooling. The most important updates include upgrading Rust from version 1.81 to 1.84 across the project, enhancing the settings.json for better development experience, and refactoring lifetimes in trait implementations for improved simplicity.

Rust version upgrade:

• Updated Rust toolchain version from 1.81 to 1.84 in .github/workflows/rust.yml, Cargo.toml, rust-toolchain, and scripts/check.sh to ensure compatibility with the latest features and improvements. [1] [2] [3] [4]
• Updated the msrv (Minimum Supported Rust Version) in clippy.toml and scripts/clippy_wasm/clippy.toml to align with the new Rust version. [1] [2]

Development tooling enhancements:

• Added new settings in .vscode/settings.json to enforce consistent formatting (formatOnSave, trimTrailingWhitespace, insertFinalNewline) and improve Rust Analyzer behavior with custom target directories and semantic token customizations.

Code refactoring:

• Simplified lifetime annotations in trait implementations by replacing explicit lifetimes (e.g., 'a) with implicit lifetimes ('_) in egui_plot/src/items/mod.rs, egui_plot/src/items/values.rs, and egui_plot/src/lib.rs. This reduces verbosity and improves readability. [1] [2] [3] [4] [5] [6] [7]

Minor improvements:

• Added unique id to markers in demo/src/plot_demo.rs to prevent potential conflicts during rendering.
• Replaced map_or with is_some_and for better clarity in linked cursor checks in egui_plot/src/lib.rs.

Documentation updates:

• Improved formatting and structure of the README.md for better readability, including converting links to angle-bracket syntax and reorganizing sections.

• Closes Toggling plot item visibility via legend is broken #99

• Closes PlotItemBase is unresolved when trying to implement trait PlotItem #101

• I have followed the instructions in the PR template

[emilk]

Very nice - thank you!