Migrate Reselect source to TS

[markerikson]

This PR:

• Renames index.js to index.ts
• Rewrites index.ts to actually be TypeScript, using all the types updates from TypeScript update for 4.x #486
• Extracts the reusable types to a new types.ts file
• Temporarily leaves the pre-migration typedefs around as src/old-typedefs.ts
• Updates the build tooling to correctly build the right artifacts while compiling TS source and generating output types, based on the setup we just added recently for React-Redux (including renaming all the build commands to match)
• Adds a Prettier config file copied from React-Redux
• Rewrites the test setup to use Jest+ts-jest instead of Mocha
• migrates the test file from JS+Mocha to TS+Jest
• Adds some additional typetests and formats the test file (hence the big diff)

[codecov]

Codecov Report

Merging #511 (82cdbbc) into master (78fc27f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #511   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         1    -1     
  Lines           21        53   +32     
  Branches         0         7    +7     
=========================================
+ Hits            21        53   +32     

Impacted Files	Coverage Δ
src/index.ts	100.00% <100.00%> (ø)
.babelrc.js