feat: new campaign to facilitate instant distribution of assets

[smol-ninja]

Changelog

• Closes Implement SablierMerkleInstant - A merkle lockup contract to facilitate instant airdrop campaigns v2-core#995
• Although this PR does not change APIs for SablierV2MerkleLL and SablierV2MerkleLT contracts, it makes some changes to them in order to allow all three Merkle contracts to share common code.

Requesting @andreivladbrg and @PaulRBerg to leave your comments if you have other suggestions.

Alternate possibilities

• If we wish to make no changes at all to the Merkle Lockup contracts, then another solution would be to duplicate the common code inside SablierMerkleInstant itself.
• If we are going to deploy SablierMerkleInstant in the next release and not prior to that, then we can also change the API structure for SablierV2MerkleLL and SablierV2MerkleLT such that Lockup specific variables (cancelable and transferable) can be removed from the common constructer parameters, as they are not relevant to Merkle Instant contract.

Pending

• Tests

[smol-ninja]

Closing this and will re-open in core repo.