Handle numbers formatted with underscores in tokenizer

[MikhailArkhipov]

Fixes #1779

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@51224e7). Click here to learn what that means.
The diff coverage is 96.96%.

@@            Coverage Diff            @@
##             master    #1819   +/-   ##
=========================================
  Coverage          ?   74.13%           
=========================================
  Files             ?      282           
  Lines             ?    13142           
  Branches          ?     2357           
=========================================
  Hits              ?     9743           
  Misses            ?     3267           
  Partials          ?      132

Impacted Files	Coverage Δ
src/client/language/tokenizer.ts	98.01% <100%> (ø)
src/client/language/characters.ts	66.17% <83.33%> (ø)

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