Fix: Allow ++ command to work with projects that don't exist yet

Problem

When you run sbt '++2.12.19 docs/publishSite', it fails with "Project not found" if the docs project only exists after switching to Scala 2.12.19.

The workaround has been to use a semicolon to separate commands: sbt '++2.12.19; docs/publishSite'. This works because the semicolon forces sbt to execute the version switch first, then parse the second command in the new state where docs exists.

Solution

The parser now accepts project/task patterns even when the project isn't available in the current state. It works like this:

1. First, try to parse the command normally using state.combinedParser (validates against current project structure)
2. If that fails, fall back to accepting any project/task pattern using a simple slash-delimited parser
3. The command validation happens later during execution, after the Scala version switch completes

By the time the command actually runs, the project exists in the new Scala version's state.

Implementation

Added a slashCommand fallback parser that matches project/task patterns:

val slashCommand = (NotSpace ~ ('/' ~> any.+)).map { case p ~ c => p + "/" + c.mkString }
val commandParser = state.combinedParser | slashCommand
val optionalCommand = Parser.opt(token(Space ~> matched(commandParser)))

The fix is targeted to only accept slash-delimited patterns, which avoids interfering with other parser components like the -v verbose flag.

Testing

The fix passes the existing actions/cross-advanced scripted test, which includes various cross-build scenarios with project-scoped commands.

Fixes #7574

Contribution by Gittensor, see my contribution statistics at https://gittensor.io/miners/details?githubId=94194147