(core) include pg package in grist-core

Summary: Grist works well with postgres@11 and earlier, when the needed TYPEORM_* environment variables are set. This includes the package needed, for convenience.

Test Plan: manual

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D3438

package.json

@@ -125,6 +125,7 @@
     "multiparty": "4.2.2",
     "node-fetch": "2.2.0",
     "numeral": "2.0.6",
+    "pg": "8.6.0",
     "plotly.js-basic-dist": "1.51.1",
     "popper-max-size-modifier": "0.2.0",
     "popweasel": "0.1.18",