In build.gradle, the dependencies on distZip and/or distTar causes
the shadowJar task to generate a second JAR file for which the
mainClass.set("seedu.duke.Duke") does not take effect.
Hence, this additional JAR file cannot be run.
For this product, there is no need to generate a second JAR file
to begin with.

Let's remove this dependency from the build.gradle to prevent the
shadowJar task from generating the extra JAR file.

This reverts commit 66ca9d2.

Add Command, Controller and InvalidCommandException classes
Add Junit Tests for delete, mark and unmark functions

Using the exit command now properly closes
javafx window after a pause.

Bot now properly saves tasks
to the task file after every change
to the task list.

Create packages 'ui', 'middleware', 'commands'
and 'exceptions' to further organise code.

Add new exception

Add assertions

Tagging tasks is unavailable.

User can't categorise tasks since description can't be changed.

Add new 'tag' string field to each task, and add new 'tag' command so
users can tag current tasks.

No help manual for the user.

Create user guide to orient the user on how to use the bot.

The user can immediately use the bot without facing errors with bot
usage.