[Flutter 3.29] Update docs and build.cs to handle Kotlin .gradle.kts files.

[timbotimbo2]

Description

Since Flutter 3.29, new projects will use the Kotin gradle DSL.
Files like build.gradle and settings.gradle now have a .gradle.kts extension.

Besides the file extension change, the syntax is different.

For example:

- implementation project(':unityLibrary')
+ implementation(project(":unityLibrary"))

flatDir {
-    dirs "${project(':unityLibrary').projectDir}/libs"
+    dirs(file("${project(":unityLibrary").projectDir}/libs"))
}

This leads to issue like #1002

This PR:

• Adds the new syntax in the README.
• Make build.cs capable of adjusting .gradle.kts files in the Unity export.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

I did not test the Unity Export Android Plugin option, as I'm not sure how that is used.
But I did update it with the same changes as the regular Android (debug/release) export, which I did test.

The example project needs to be updated before it will be compatible with Flutter 3.29.
For a new Flutter 3.29 project this PR is enough.

With Flutter 3.29 you will likely have to define ndkVersion in app/build.gradle.kts.
In my case, the Flutter console error messages told me to use ndkVersion = "27.0.12077973".