Fix underscore identifiers and add local plugin build workflow

[Dokiys]

Summary

• Fix editor selection so underscore-separated identifiers such as vendor_account_id can be converted correctly. Cycling between cases doesn't work properly anymore #35
• Add a Gradle IntelliJ Platform build setup and Gradle Wrapper.
• Add make build for producing an installable plugin ZIP.
• Add optional local build configuration for IDE paths and network mirrors without committing machine-specific settings.
• Document Marketplace installation and installing a locally built ZIP from disk.

Validation

• make build