Add helpers to find and add suppressions comments to nodes

[amyreese]

Stack from ghstack (oldest at bottom):

• -> Add helpers to find and add suppressions comments to nodes #451
• Extract node_comments to new module, return nodes instead of strings #450

[zsol]

You'll need to add typing extensions to Fixit's dependencies now (it was only pulled in by libcst for up to 3.8)

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