Add Zsh support to Spin environment file

The previous commit made it possible to source the Spin environment file
from a Bash script, but we also need to be able to source it directly
from a Zsh command line. `$BASH_SOURCE` isn’t supported by Zsh so the
current solution doesn’t work for the latter purpose.

This commit uses shell parameter expansion syntax [0,1] to supply a
Zsh-compatible equivalent if `$BASH_SOURCE` isn’t set.

The expression `${(%):-%x}` in Zsh does the same job as `$BASH_SOURCE`
in Bash: `(%)` is a parameter expansion flag [2] which enables prompt
sequence expansion [3] in what follows, and `%x` is the prompt sequence
for “the name of the file containing the source code currently being
executed” [4].

[0] https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
[1] https://zsh.sourceforge.io/Doc/Release/Expansion.html#Parameter-Expansion
[2] https://zsh.sourceforge.io/Doc/Release/Expansion.html#index-parameter-expansion-flags
[3] https://zsh.sourceforge.io/Doc/Release/Prompt-Expansion.html
[4] https://zsh.sourceforge.io/Doc/Release/Prompt-Expansion.html#Shell-state

Author: tomstuart

.spin/bin/env

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-SCRIPT_PATH=$(dirname "$BASH_SOURCE[0]")
+SCRIPT_PATH=$(dirname "${BASH_SOURCE[0]:-${(%):-%x}}")
 SCRIPT_PATH=$(cd "$SCRIPT_PATH" && pwd)
 TRUFFLERUBY_DIR=$SCRIPT_PATH/../..
 export JT_ENV=jvm-ce