Migrate plugin to current IntelliJ Platform template and add Rider su…

[guiszlima]

Migrate Omni Theme plugin to the current JetBrains plugin template

Description

This PR migrates the Omni Theme plugin to the current JetBrains plugin structure.

The theme itself remains unchanged. The goal of this change is to update the project structure and configuration so the plugin can build correctly and work with current versions of JetBrains IDEs.

Changes made

• Migrated the project structure to match the current JetBrains plugin template.
• Updated plugin configuration and resources to follow current IntelliJ Platform requirements.
• Adjusted the project setup to generate a valid plugin artifact for modern JetBrains IDE versions.
• Updated the build configuration to work with the current plugin tooling.

Validation

The plugin was tested using:

./gradlew build

and:

./gradlew verifyPlugin

The Plugin Verifier completed successfully:

BUILD SUCCESSFUL

Motivation

I have been using Omni Theme for several years and wanted to contribute by keeping the plugin compatible with the latest JetBrains IDE versions.

The visual appearance of the theme is unchanged; this PR only updates the underlying plugin structure and compatibility.