Add modular FCL declaration types (Flow dev grant 18, milestone 1)

[louisguitton]

ref : https://github.com/onflow/developer-grants/issues/18

Legend: 🥖 = bread crumbs = I review myself to point reviewers or other committers to points of attention

TODO

• [x] Port changes from https://github.com/louisguitton/DefinitelyTyped/pull/1 here using modular declaration files
• [ ] test end to end by "npm install @onflow/fcl" but using this branch's version
• [ ] include remaining feedback from the discussion in https://github.com/louisguitton/DefinitelyTyped/pull/1
• [ ] review https://github.com/samatechtw/onflow-fcl-esm and include as much as possible

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