Start deprecating 'var' in favor of 'let'.

src-electron/generator/generation-engine.js

src-electron/generator/helper-c.js

src-electron/generator/helper-endpointconfig.js

src-electron/generator/helper-sdkextension.js

src-electron/generator/helper-session.js

src-electron/generator/helper-zap.js

src-electron/generator/helper-zcl.js

src-electron/generator/overridable.js

src-electron/generator/template-engine.js

src-electron/generator/template-util.js