Ensure that child build processes started when running on Windows exi…

…t properly.

plugin-run-script and plugin-build script use execa.command with the shell: true
option set to execute the user's command.  On Windows, this causes the child process
to remain running after Snowpack exits: sindresorhus/execa#433.
The fix is to override the default windowsHide option -- as Snowpack always runs from
a console we don't actually need execa to hide any extra console windows for us.

plugins/plugin-build-script/plugin.js

@@ -21,6 +21,7 @@ function buildScriptPlugin(_, {input, output, cmd}) {
         env: npmRunPath.env(),
         extendEnv: true,
         shell: true,
+        windowsHide: false,
         input: contents,
         cwd,
       });

plugins/plugin-run-script/plugin.js

@@ -13,6 +13,7 @@ function runScriptPlugin(_, {cmd, watch, output}) {
         env: npmRunPath.env(),
         extendEnv: true,
         shell: true,
+        windowsHide: false,
         cwd,
       });
       const {stdout, stderr} = workerPromise;