feat: export fallback from fallback (circular)

[alexander-fenster]

Turns out our code is not perfectly fit for Electron because Electron's client side, even though it's technically a browser, does not respect "browser": field in package.json, so it does not get redirected to build/src/fallback.js automatically. This was reported here: googleapis/nodejs-speech#547

The workaround for this is to prepare a bundle first (with either webpack or browserify), but long term solution would be to change fallback detection logic in the client a little bit. To do it safely, I need this circular export:

> const gax = require('google-gax')
undefined

> gax
{
    ...
    fallback: {
        ...
    }
}

> gax.fallback
{
    ...
    fallback: [Circular]
}

> gax.fallback.fallback
{
    ...
    fallback: [Circular]
}

With this change, we'll be safely forcing users to gax.fallback in the client code if window is defined, without caring much if the "browser": field worked or not.

[codecov]

Codecov Report

Merging #803 into master will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #803      +/-   ##
==========================================
- Coverage   90.02%   89.89%   -0.14%     
==========================================
  Files          48       48              
  Lines        8113     8123      +10     
  Branches      479      525      +46     
==========================================
- Hits         7304     7302       -2     
- Misses        806      818      +12     
  Partials        3        3              

Impacted Files	Coverage Δ
src/fallback.ts	77.64% <100.00%> (+0.53%)	⬆️
.mocharc.js	78.57% <0.00%> (-14.29%)	⬇️
src/normalCalls/retries.ts	93.58% <0.00%> (-5.13%)	⬇️

---

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 cc99de0...0a2ea58. Read the comment docs.

[alexander-fenster]

Can I ping you on this @bcoe @JustinBeckwith ? Useful for Electron.