Add support for running snippets

[Gedochao]

Fixes #1150

Additionally, this allows to run Scala and Java snippets in a similar fashion as -e allows for scripts.
Context:

• -e/--script-snippet/--execute-script allows for running code as a .sc script; it's similar to piping code via stdin to _.sc
  • this is meant to be the equivalent of scala -e

$ scala-cli -e 'println("Hello")'
Hello

• --scala-snippet/--execute-scala allows for running raw .scala code; it's similar to piping code via stdin to _.scala

$ scala-cli --scala-snippet '@main def run() = println("Hello")' 
Hello

• --java-snippet/--execute-java allows for running raw .java code; it's similar to piping code via stdin to _.java

$ scala-cli --java-snippet 'public class Main { public static void main(String[] args) { System.out.println("Hello"); } }'
Hello

[alexarchambault]

Looks great!

Two remarks:

• it seems the --script-snippet option and all can only be specified a single time each. Is that intended? (to be inline with scala -e maybe?) Changing their types to List[String] would allow them to be specified multiple times, which could be handy.
• I think it might be possible to preserve the order of these arguments, and add them to the other inputs in the right order too (relative to the other inputs), by using caseapp.core.Indexed, like scriptSnippet: List[Indexed[String]]. We get the other inputs via caseapp.RemainingArgs, which also offers the indices of individual arguments. That way, both --*-snippet options and other inputs would be taken into account in the right order, when computing the default main class for example. (Beware that I didn't use Indexed much, I hope it works fine…)

These points can be addressed or discussed in later PRs if needed, IMO.

[Gedochao]

• it seems the --script-snippet option and all can only be specified a single time each. Is that intended? (to be inline with scala -e maybe?) Changing their types to List[String] would allow them to be specified multiple times, which could be handy.

Initially I intended to have it as close to scala's -e as possible, but later it became clear that it should at the very least support whatever we have for piped inputs. Supporting a list of snippets passed my mind, but I wanted to get some feedback first. I'll add it in a separate PR.

• I think it might be possible to preserve the order of these arguments, and add them to the other inputs in the right order too (relative to the other inputs), by using caseapp.core.Indexed, like scriptSnippet: List[Indexed[String]]. We get the other inputs via caseapp.RemainingArgs, which also offers the indices of individual arguments. That way, both --*-snippet options and other inputs would be taken into account in the right order, when computing the default main class for example. (Beware that I didn't use Indexed much, I hope it works fine…)

Hm... the potential benefits of doing this are minimal, if I understand correctly... But I'll try it in a separate PR