Use better path manipulation, update plugin file name

@nunico mentioned a better way to massage `$PATH`, use it instead.

Rename iterm2-shell-integration.plugin.zsh -> iTerm2-shell-integration.plugin.zsh
per @nunico's comments about issues with zimfw and potentially other plugin managers.

Signed-off-by: Joe Block <[email protected]>

Author: unixorn

iterm2-shell-integration.plugin.zsh renamed to iTerm2-shell-integration.plugin.zsh

@@ -1,8 +1,7 @@
 # Automagically add the utilities directory to $PATH
 
 # Add utilities to our path
-PLUGIN_BIN="$(dirname $0)/utilities"
-export PATH=${PATH}:${PLUGIN_BIN}
+path+=("${0:h}/utilities")
 
 # And source the helper functions, too.
 source "${0:h}/shell_integration/zsh"