Chore: update way of building

[JoviDeCroock]

I've updated the way of building to a more conform approach to standards and the way discussed in apollo-client
apollographql/apollo-client#4261

This covers a

• tree-shakeable esm bundle
• jsnext:main is unknown to most bundlers so I removed it
• an exploded cjs build

[codecov-io]

Codecov Report

Merging #918 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #918   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files          20       20           
  Lines         662      662           
  Branches      147      147           
=======================================
  Hits          638      638           
  Misses         19       19           
  Partials        5        5

---

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[codecov-io]

Codecov Report

Merging #918 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #918   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files          20       20           
  Lines         662      662           
  Branches      147      147           
=======================================
  Hits          638      638           
  Misses         19       19           
  Partials        5        5

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fbfa9a8...817a3c4. Read the comment docs.

[hwillson]

Looks great @JoviDeCroock - thanks!