Gradle update

[FalsePattern]

Gradle 9.0 is releasing soon, with the configuration cache enabled by default, so I updated the plugin's buildscript to be compatible with it, along with some other minor tweaks.
The changes are:

• Added an explicit gradle wrapper. This way the build is independent from the system-installed gradle version
• Translated the build.gradle file from groovy to kotlin, as modern Intellij versions have better highlighting/completion support for it
• Reimplemented the generateGrammarClean task to be config cache compatible by breaking out each generation step (lexer, highlight lexer, parser) into a separate sub-task with its own dedicated generation target directory. This way the source sets are not mixed, and changing one grammar does not require rebuilding the others.
• Enabled the gradle configuration cache
• Disabled the gradle kotlin stdlib dependency, because the kotlin stdlib is provided by the IDE. This is recommended by the plugin SDK documentation: https://plugins.jetbrains.com/docs/intellij/using-kotlin.html#kotlin-standard-library
• Migrated the intellij task configurations to the intellijPlatform {} project DSL. This is functionally equivalent, but the platform plugin has extra safety checks on these.
• Added the plugin verifier, can be invoked with the verifyPlugin gradle task. This is the same verifier that they use on the marketplace.