🥑 Add mock contract typing

[pawelpolak2]

closes #765

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 807e05890b6183aababba173d15dc26a40aceecb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

| Name | Type | | ------------------------------ | ----- | | @ethereum-waffle/mock-contract | Patch | | ethereum-waffle | Patch |

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