snapcraft trggers unnecessary pulls from unrelated file modifications

[Aleksandr Bogdanov]

when working in an InteliJ IDEA based IDE or VSCode, or even simply having changes in .git directory, `snapcraft` marks stages as stale, which causes unnecessary re-pulls and re-builds, which slows down e.g npm packaging.

I propose to add .snapignore file support, to allow developers to exclude files included in a snap and as such known to not invalidate packaging steps.