Build script switch to generate either shell xor binary for bin/enso

[JaroslavTulach]

The instant launching of Enso programs discussion describes needs for:

• Change build script to not include shell based launchers (e.g. no enso.bat in release)
• Include native image launcher (without -ea option) in the release

Development

• Don't build native image launcher for developers by default, use shell based launchers
• Allow building native image launcher at explicit request

All these requests have one in common:

• the current behavior or buildEngineDistribution that generates shell launchers
• followed by engine-runner/buildNativeImage that adds/overrides bin/enso or bin/enso.exe
• is confusing, for example:
• it is used in the CI and caused some cleanup mismatches (eliminated by now)
• we need a switch to tell the build script whether to behave as of now or do only engine-runner/buildNativeImage

The Switch

An environment variable would be a good switch and would play well with the CI as the case of ENSO_JAVA shows. Let's thus introduceENSO_LAUNCHER which can have following values

• native
• debugnative
• shell

based on these values it either executes the current copy of shell launchers or it skips this behavior and invokes engine-runner/buildNativeImage instead. The debugnative variant shall include -ea flag when invoking native-image - a mode needed for running Enso tests.

After the change it should be possible to:

enso$ ENSO_LAUNCHER=native sbt buildNativeImage
enso$ ls -l ./built-distribution/enso-engine-*/enso-*/bin/enso*

and see either only enso Unix executable or enso.exe on Windows.

Notes

• the default value for ENSO_LAUNCHER remains shell for now - e.g. release behavior remains unchaged
• change CI to use the ENSO_LAUNCHER=debugnative variable instead of invoking engine-runner/buildNativeImage directly
• update documentation