script extension can be either '.sc' or empty string

[philwalk]

File with no extension only considered a script if file has a shebang header.
This is a follow-up to #3794 which passed CI tests but implemented a less restrictive script definition.

A script with a valid shebang header and no extension is recognized as a valid script even when launched from a shell that isn't shebang capable. Consequently, it can be run when directly passed to scala-cli without the shebang command, just like a script with '.sc' extension.

A side-effect of the less restrictive script definition in #3794 is that the wrapper class for a script with .scala extension also has a def scriptPath defined, which is nice. However, without knowing all the potential side-effects of that change, it seems safer to restrict the scope here.

[philwalk]

@Gedochao - At least some of these test failures are download problems:

download error: Caught java.io.IOException (Server returned HTTP response code: 503 for URL: https://github.com/graalvm/graalvm-ce-builds/releases/download/vm-22.3.1/graalvm-ce-java17-linux-amd64-22.3.1.tar.gz) while downloading https://github.com/graalvm/graalvm-ce-builds/releases/download/vm-22.3.1/graalvm-ce-java17-linux-amd64-22.3.1.tar.gz

Run VirtusLab/scala-cli-setup@v1
Install Coursier
Error: Unexpected HTTP response: 503

All but 1 of the 78 failures are passing on my Linux box.
The one failing test is `'sclicheck.GifTests.complete-install' which times out.

Reading the first 2 bytes of a script file to compare against #! is problematic if the file is a pipe or a stream, so it's been disabled when when processing all files in a directory. I'm not quite sure I fully understand that use-case, so I don't know if that will be a problem.

Let me know if I should squash everything to a single commit.

[Gedochao]

LGTM
I'll squash the commits when merging, no worries.

[philwalk]

The 2 failing tests pass on my aarch64 mac f.w.i.w.

• 'scala.cli.integration.BspTests213.setup-ide doesn't pass unrecognised arguments to old --cli-versions: 1.5.0-19-g932866db6-SNAPSHOT'
• 'scala.cli.integration.BspTests213.setup-ide prepares a valid BSP configuration with --cli-version 1.5.0-34-g31a88e428-SNAPSHOT'

[Gedochao]

Suspecting intermittent failures on the CI, irrelevant to this PR...