refactor(app): rename builtinExtensions to builtinFeatureTiles + drop dead extensions table from datastore browser
app/index.js
- rename builtinExtensions array to builtinFeatureTiles
- rename registerExtensionCommands to registerCoreCommands
- refresh adjacent comments to use tile/feature vocabulary
app/settings/settings.js
- drop the deleted extensions SQLite table from the Datastore browser table-groups list
- rename that group's label from "Extensions" to "Features"
- drop the corresponding tableDescriptions entry