Add a Snapcraft Build and Snap Install Instructions

[sankalpgambhir]

Adds the folder snap/ containing a snap build file and two scripts to launch stainless and stainless.cli in the appropriate container environment.

The build is more manual scripting than one would like, but Snap has no Scala/sbt plugin/build, only Maven/Gradle.

Once the build files are merged, we can also try and set up automatic builds to the Snap store when we commit to main.

The following section is inserted into the README right above the AUR install instructions:

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Snap Store

A package for Stainless is available on the Snap store as stainless with an experimental edge release. It can be used to install and run Stainless on any Snap enabled system (e.g. Ubuntu).

In a terminal, you can type:

sudo snap install stainless --edge

This exposes two commands, the tool stainless, as well as stainless.cli, running scala-cli with Stainless libraries loaded.

Running the commands the first time may take some time as some Scala libraries are downloaded.

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