Universal shell scripts to run baselisk

What do you think about having files with native shell scripts like [gradle wrapper](https://docs.gradle.org/current/userguide/gradle_wrapper.html) have? Somthing like this:

```bash
#!/usr/bin/env sh

##############################################################################
##
## Bazel start up script for UN*X (based on gradlew script from gradle wrapper project)
##
##
##############################################################################

# Attempt to set APP_HOME
# Resolve links: $0 may be a link
PRG="$0"
# Need this for relative symlinks.
while [ -h "$PRG" ] ; do
ls=`ls -ld "$PRG"`
link=`expr "$ls" : '.*-> \(.*\)$'`
if expr "$link" : '/.*' > /dev/null; then
PRG="$link"
else
PRG=`dirname "$PRG"`"/$link"
fi
done
SAVED="`pwd`"
cd "`dirname \"$PRG\"`/" >/dev/null
APP_HOME="`pwd -P`"
cd "$SAVED" >/dev/null

APP_NAME="bazelisk"
APP_BASE_NAME=`basename "$0"`

warn () {
echo "$*"
}

die () {
echo
echo "$*"
echo
exit 1
}

## Determine OS.
darwin=false
linux=false

case "`uname`" in
Darwin* )
darwin=true
;;
Linux* )
linux=true
;;
esac

BINARY_PATH=""
if $darwin; then
BINARY_PATH=bazelisk-darwin-amd64
elif $linux; then
BINARY_PATH=bazelisk-linux-amd64
else
die "UNSUPPORTED OS! Command 'uname' says: `uname`. 'darwin' = $darwin and 'linux' = $linux"
fi

exec "$APP_HOME/bazelisk/$BINARY_PATH" "$@"
```

while having bazelisk binaries inside VCS repo?

Start with the proposed Unix shell script and the linked Gradle wrapper documentation. Compare that approach with Bazelisk’s launcher and binary layout. Done means the project has a settled scope for native shell launchers and for how the Bazelisk binaries are kept in or obtained from version control.

Written by the indexing model from the issue text.