converted event listeners reducer to es modules

[Erica42]

Associated Issue: #1884

Summary of Changes

• changed to import from require and added export

Test Plan

Tell us a little a bit about how you tested your patch.

Example test plan:

• Command-P opens the panel
• Clicking “+” opens the panel
• Clicking a source navigates to the source
• Clicking "x" closes the panel

Here's the Debugger's Testing doc
https://docs.google.com/document/d/1oBMRxV8A2ag2t22YsQOxTdEv0mXKzIg0tggJjRkU1S0/edit#.
Feel free to improve it!

Screenshots/Videos (OPTIONAL)

[jasonLaster]

Thanks @Erica42! If the tests are green I'll merge it.

Any luck with webdriver?

[codecov]

Codecov Report

Merging #2473 into master will decrease coverage by 0.15%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2473      +/-   ##
==========================================
- Coverage   54.54%   54.39%   -0.16%     
==========================================
  Files          52       52              
  Lines        2013     2015       +2     
  Branches      392      391       -1     
==========================================
- Hits         1098     1096       -2     
- Misses        915      919       +4

Impacted Files	Coverage Δ
src/reducers/event-listeners.js	22.22% <50%> (-7.78%)	⬇️
src/utils/editor/index.js	11.11% <0%> (-0.76%)	⬇️

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[jasonLaster]

I'm okay with losing some of the code coverage here. I'm not even sure why this would happen :)