Support branching in static analysis

[guybedford]

This fixes #235 in supporting asset analysis on statements like:

`const filename = `${__dirname}/fonts/Inter-UI-${fontWeight === 'bold' ? 'Bold' : 'Regular'}.woff2`;`.

The static evaluator from https://github.com/substack/static-eval is forked and updated to support two branches throughout the analysis paths (any more branches and we go back to an unknown deoptimization state).

Thus even more complex compound cases like __dirname + '/' + (unknown ? 'a' : 'b') + (__dirname ? 'a' : 'b') all get supported fine.

Thanks @styfle for the issue, it's these observations and requests that allow the analysis to develop :)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@04fbbe7). Click here to learn what that means.
The diff coverage is 53.77%.

@@            Coverage Diff            @@
##             master     #255   +/-   ##
=========================================
  Coverage          ?   64.51%           
=========================================
  Files             ?       17           
  Lines             ?      854           
  Branches          ?        0           
=========================================
  Hits              ?      551           
  Misses            ?      303           
  Partials          ?        0

Impacted Files	Coverage Δ
src/loaders/relocate-loader.js	97.23% <100%> (ø)
src/utils/static-eval.js	43.67% <43.67%> (ø)

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