Disable with DISABLE_SPRING_WATCHER_LISTEN env var

We have a project where some Engineers are running configurations that
do not support `inotify`, which causes an error when this gem is loaded.
We still want these Engineers to be able to use Spring in polling mode.
We tried setting `require: false` in the `Gemfile` but [Spring loads the
gem
anyway](https://github.com/rails/spring/blob/a318a1837524ea6e39a7073af94e75131fcc95d1/lib/spring/commands.rb#L35-L46).

This change introduces a new env var `DISABLE_SPRING_WATCHER_LISTEN` which,
along the lines of `DISABLE_SPRING`, disables the functionality in this
gem.

Author: f1sherman

README.md

@@ -14,6 +14,11 @@ On larger projects this means spring will be more responsive, more accurate and
 Listen 2.7 and higher and 3.0 are supported.
 If you rely on Listen 1 you can use v1.0.0 of this gem.
 
+## Environment variables
+
+* `DISABLE_SPRING_WATCHER_LISTEN` - If set, this disables the loading of this gem. This can be useful for projects where
+    some configurations do not support inotify (e.g. Docker on M1 Macs).
+
 ## Installation
 
 Stop Spring if it's already running:

lib/spring/watcher/listen.rb

@@ -1,3 +1,5 @@
+return if ENV['DISABLE_SPRING_WATCHER_LISTEN']
+
 require "spring/watcher"
 require "spring/watcher/abstract"