docs: add typography changes to migration guide

[NickyMeuleman]

Add info about changes to the typography utils file to the migration guide for moving from v1 to v2.

edit: woah, hold on, seeing changes in that file I didn't make (the what we'll cover section)
A markdown extension in my editor changed the table-of-contents, changed it back, we're all good now!

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[m-allanson]

Thanks @NickyMeuleman! Apologies for the delay reviewing this. I'm a bit hesitant to include the change like this as I think (although I'm not certain) that this is related to using babel 7 rather than being a plugin specific change.

As this is definitely useful information for people upgrading their sites, I'm going to merge as-is and maybe we can revisit it later.

[jlengstorf]

Hiya @NickyMeuleman! 👋

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