New runDevSlim alternative to "dev"

[dsmiley]

This introduces a convenient way to start Solr from Gradle, which I think is thus the most convenient way to start Solr period. It relies on devSlim. Note that IntelliJ can run Gradle tasks (action "Execute Gradle Task"), which in turn makes debugging simple, as you just need to hit the debug symbol.

Downsides / trade-offs:

• skips bin/solr (bash), thus isn't in sync with obscure settings it sets. Not going to address that at all; since being a "JavaExec" task is quite the point, and is what benefits from being able to debug quickly.