Originally, the version check TTL file was located alongside the CDK
node installation directory. This caused issues for people who had CDK
installed on a readonly filesystem.

This change moves this file to the user's home directory, specifically
under $HOMEDIR/.cdk/cache. This directory is already being used by
account-cache.ts.

The old logic is maintained where a repo check is run once a day against
NPM, when a CDK command is invoked, and when a new version is present
will generate a console banner notifying the user of this.

The edge case with the updated implementation is when there are 2+
active installations of CDK and they are used interchangably. When
this happens, the user will only get one notification per non-latest
installation of CDK per day.

* Drop use of fs and use fs-extra exclusively.
* Cleaned up file write.
* Fixed breaking unit test.

#2774