Make input not required

[PaulRBerg]

This PR makes the version input non-required, defaulting to nightly. I think that this is a sensible thing to do now, given that Foundry is in active development and only has nightly releases. We might revert back to this later on when Foundry starts releasing stable versions as well.

Full changelog:

• refactor: make "version" input not required
• chore: use double quotes in "action.yml"
• docs: add brief description about Foundry in README
• docs: fix typos in README
• docs: remove optional input "version" from example script
• refactor: set "nightly" as default value for "version"

[PaulRBerg]

Thanks for accepting this PR - though we can close this because #19 made the input optional.