This commit introduces a convention plugin for multiplatform projects to centralize common configuration.

The plugin sets up the JVM toolchain, configures JVM and JS targets, and adds basic dependencies for common, JVM, and JS source sets.

Individual module build scripts now use the `kotlinMultiplatformConvention` plugin to apply this shared configuration.

I submit my source code and allow Tigase to release the code as a part of Tigase's own software package under any license, either an open source or closed source.

(cherry picked from commit fa9a1c0f9ab63801f002e626fc3979947a044686)

I submit my source code and allow Tigase to release the code as a part of Tigase's own software package under any license, either an open source or closed source.