Make `express` an optional peer dependency

[benmccann]

express is a very heavy library with 67 dependencies: https://npmgraph.js.org/?q=express

It doesn't seem to be used in a required functionality here, but in optional functionality:

bolt-js/src/receivers/ExpressReceiver.ts

Line 26 in 7c0ed4f

} from 'express';

Users who are using express will have it installed. Those who are using a different web server should not have express installed

Requirements

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