fix: debugger statements lack sourcemaps

[connor4312]

Fixes #501
Fixes microsoft/vscode-js-debug#954

This fixes the linked issue in js-debug, since because the debugger statement didn't have a sourcemap, we were showing paused at the previous, closest position in the source.

I also realized when I added tests in #354, I forgot to actually add the assertion that mappings existed. Oops 😄

[codecov]

Codecov Report

Merging #502 (e7e9d7c) into master (d3e22f9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #502   +/-   ##
=======================================
  Coverage   94.97%   94.97%           
=======================================
  Files           1        1           
  Lines        1134     1134           
=======================================
  Hits         1077     1077           
  Misses         57       57           

Impacted Files	Coverage Δ
src/astring.js	94.97% <100.00%> (ø)

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[davidbonnet]

Thanks @connor4312 for reporting and handling this issue.
Looks good but an extra fixture folder isn't required. I'll update.