Fix empty dist folder in npm pacakge (#9125)

Use a .npmignore file instead of the files property of package.json

A bug in lerna seems to use the generated dist/package.json instead of the root file
when determining files to add, resulting in an empty directory.

Related: https://stackoverflow.com/questions/54912262/lerna-publish-and-npm-pack-failing-to-package-all-the-files-in-dist-folder

packages/backend-core/.npmignore

@@ -0,0 +1,2 @@
+*
+!dist/*

packages/backend-core/package.json

@@ -1,8 +1,5 @@
 {
   "name": "@budibase/backend-core",
-  "files": [
-    "dist"
-  ],
   "version": "2.2.10-alpha.2",
   "description": "Budibase backend core libraries used in server and worker",
   "main": "dist/src/index.js",