Drop special behavior for legacy browsers

[adamwathan]

This PR removes the special handling of the : character in CSS identifiers so that they simply escaped as \:, and also removes the special handling of leading _ characters in identifiers so they are not escaped.

Hopefully I've followed the build process properly, I noticed there were changes in src/cssesc.js that hadn't been added to the build in /cssesc.js (namely the version number) so those changes were sort of brought in accidentally just by me running the build.

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 1.69%.
The diff coverage is 83.33%.

@@           Coverage Diff            @@
##           master     #19     +/-   ##
========================================
- Coverage     100%   98.3%   -1.7%     
========================================
  Files           1       1             
  Lines          63      59      -4     
  Branches       19      17      -2     
========================================
- Hits           63      58      -5     
- Misses          0       1      +1

Impacted Files	Coverage Δ
cssesc.js	98.3% <83.33%> (-1.7%)	⬇️

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