chore: set save-exact for npm

started using exact versions originally when
attempting to use renovate. seemed like a decent idea.

adding this now because it was annoying to npm install and have to
remove the carrot for consistency every time!

.npmrc

@@ -2,3 +2,5 @@
 engine-strict=true
 # workaround for getting storybook to work with npm7+; issue closed but hopefully this is addressed eventually https://github.com/storybookjs/storybook/issues/18298#issuecomment-1209929821
 legacy-peer-deps=true
+# don't upgrade package versions unintentionally
+save-exact=true