refactor(types): refactors index to export all types

[steffnay]

Fixes #456

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #548 into master will increase coverage by <.01%.
The diff coverage is 98.55%.

@@            Coverage Diff             @@
##           master     #548      +/-   ##
==========================================
+ Coverage   99.42%   99.43%   +<.01%     
==========================================
  Files           5        6       +1     
  Lines         698      707       +9     
  Branches      192      192              
==========================================
+ Hits          694      703       +9     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
src/dataset.ts	100% <ø> (ø)	⬆️
src/table.ts	100% <ø> (ø)	⬆️
src/job.ts	100% <ø> (ø)	⬆️
src/index.ts	100% <100%> (+1.49%)	⬆️
src/bigquery.ts	98.47% <98.47%> (ø)

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[bcoe]

@stephenplusplus I believe @callmehiphop might be away for a little bit? mind providing some review.