fix: Npm ignores files and folders named "core" by default (#1767)

Since the release of `npm@6.8.0`, a version of `npm-packlist` is
included that introduces a behavior to ignore anything named "core" by
default.

Added `!core` to the `.npmignore` file as a work around, and also more
strictly pinned the `npm` version to auto-upgrade to so we don't get
surprised in this kind of way anymore.

Cause: npm/npm-packlist#24

install.sh

@@ -5,7 +5,7 @@ export NODE_OPTIONS="--max-old-space-size=4096 ${NODE_OPTIONS:-}"
 
 if ! npm ci --help; then
   echo "upgrading npm, because "npm ci" is not supported"
-  npm i -g npm
+  npm i -g npm@~6.8.0
 fi
 
 echo "============================================================================================="

packages/@aws-cdk/cdk/.npmignore

@@ -13,4 +13,7 @@ dist
 # Include .jsii
 !.jsii
 
-*.snk
+*.snk
+
+# Work around npm@6.8.0 ignoring 'core' by default -- https://github.com/npm/npm-packlist/issues/24
+!core