Add Filer.fs to support creating a filesystem like node.js

[humphd]

To be more in sync with how node.js works, this adds Filer.fs, which lazily creates a filesystem instance. It also adds Filer.path to match what node devs would expect, and allows users to do the following:

const { fs, Buffer, path } = require('filer');

I have kept Filer.Path to not break the API, but we could remove it later if we wanted.

I also spotted a stray WebSQL reference in the tests, which this also removes.

[codecov-io]

Codecov Report

Merging #651 into master will decrease coverage by 0.06%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #651      +/-   ##
==========================================
- Coverage    86.6%   86.54%   -0.07%     
==========================================
  Files          17       17              
  Lines        1740     1746       +6     
==========================================
+ Hits         1507     1511       +4     
- Misses        233      235       +2

Impacted Files	Coverage Δ
src/index.js	57.14% <57.14%> (-42.86%)	⬇️
src/filesystem/interface.js	93.52% <0%> (+0.58%)	⬆️

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[humphd]

NOTE: the lower coverage on src/index.js is because we can't run this in our current coverage environment (node) because it depends on indexeddb--it's fine.