Dynamically populate regex and java inputs

[ChipWolf]

Right now, using the action requires a regex input to targer the file that gets downloaded by:

java -jar headlessmc-launcher-${{ inputs.hmc-version }}.jar --command ${{ inputs.modloader }} ${{ inputs.mc }} --java ${{ inputs.java }}

This feels a bit clunky and could be simplified for better UX. Here are a couple of ideas:

• Make headlessmc-launcher more machine-friendly:

  It could output something like JSON or write a file with the info about the downloaded file (e.g., a manifest).

• Smarter file detection in the action itself:

  The action could scan the directory for new files to figure out what’s been downloaded.

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We also require users to run the setup-java action first, which means the runner already knows which version of Java is set up. Instead of requiring users to explicitly pass the java input, we could dynamically detect the version from the environment. Further simplifying use of the action.

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In both instances, there will be edge cases, so we should keep the inputs available as optionals.