Support for Sanskrit and Pali, especially ṁ

The ISO standard for Sanskrit/Pali requires ṁ, which is still missing for Source Serif. There are very few quality fonts for ancient Indian languages, and in the free world, none with Source's qualities.

The blog post for [Source Serif 4](https://blog.adobe.com/en/publish/2021/03/04/source-serif-gets-optical-sizes.html) indicates that AL-5 support is upcoming, which includes ṁ. Yay! So +1 for this! :+1:

Meanwhile, thanks to everyone who has made Source Serif happen. I'm using variable fonts with optical sizing on the web. For free! And it just works! Amazing how far we've come. :pray: