Add Gradle file support & format build file

[badsyntax]

Refs #185

To enable Gradle file support in spotless:

apply plugin: 'groovy'
spotless {
  groovyGradle {
    target '*.gradle' // default target of groovyGradle
    greclipse()
  }
}

To enable in vs code, add to settings.json:

"[gradle]": {
  "editor.defaultFormatter": "richardwillis.vscode-spotless-gradle",
  "spotlessGradle.diagnostics.enable": true,
  "spotlessGradle.format.enable": true
}