Development mode warnings use array.prototype.includes which breaks bundle on IE 11

[AndrewHoldaway]

Describe the bug
When bundling in development mode the check for unknown props utilizes the Array.prototype.includes method which isn't supported on ie 11. This causes a break on ie11

Logs

Expected behavior
Conduitry suggested in the discord that this should be replaced with indexOf to avoid having to use a polyfill

Information about your Svelte project:

• Your browser and the version: (IE 11)

• Your operating system: (Windows 7)

• Svelte version (3.12..1)

• Whether your project uses Webpack or Rollup

Severity
Renders usage in IE 11 while developing useless so decently annoying. Requires excess polyfills simply to develop although it is not a blocking issue as adding a polyfill should fix the issue.

[Conduitry]

caniuse says that IE doesn't support startsWith either, so we should replace that with something else as well.

[AndrewHoldaway]

This still seems to be used when dev is set to false with rollup plugin svelte

I was mistaken. Aggressive caching from the browser.

[Conduitry]

In 3.14.1, we're now using indexOf instead of includes and slice instead of startsWith. Thanks!