SDK Non-Functional Requirement improvements

[aaitor]

Is your feature request related to a problem? Please describe.

• Add ability to run tests in the browser
• Consider switching the test framework from mocha to jest ( + puppeter)
• Add missing methods to nevermined Nfts721 class (name(), symbol(), burn())
• Turn more strict linter avoiding unused code and any types (with an exception in tests)
• Re-enable node14 tests in the CI #328
• One should be able to use a github branch of this repo in package.json. This issue might be easier to solve once the exports are handled properly

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