feat: add IntelliJ IDEA plugin for generating with bzlgen

Adds OSS version of bzlgen IntelliJ IDEA plugin for bzlgen

closes #47

TODO:

• [ ] add instructions to readme for installation etc
• [ ] update ev sync to handle the new folder paths
• [ ] add tests, from quick look at the docs, this seems a pain point

Currently bzlgen needs to be installed globally for this to work, or on the PATH somewhere, I wonder if we can rollup bzlgen and bundle it in the jar, at least then the plugin and binary are versioned in step

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