feat: add ListTile.tileGradient

[jacksonb-cs]

Add the ability to provide a gradient to ListTile.

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[gemini-code-assist]

Code Review

This pull request introduces a tileGradient property to ListTile and ListTile.selectable to support gradient backgrounds. It also updates the internal container decoration to apply this gradient. The reviewer feedback suggests ensuring the mutual exclusivity assertion for tileColor and tileGradient is applied consistently across constructors, and recommends removing the unnecessary late keyword from local variables in the builder to leverage compile-time definite assignment analysis.

[bdlukaa]

Instead of a gradient, what about allowing a Decoration object? Like Container.color and Container.decoration. It'll give more flexibility and avoid adding a new parameter for each decoration parameter needed.