Redefine punctuation regex

[kbenoit]

Changes the punctuation regex from \\p{P}+ to \\p{P}. Means we correctly recognise as a username @_.

Solves #2048.

[codecov]

Codecov Report

Merging #2049 (19b4af6) into master (f680dcd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2049   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files          70       70           
  Lines        3970     3970           
=======================================
  Hits         3778     3778           
  Misses        192      192           

Impacted Files	Coverage Δ
R/tokens.R	99.55% <100.00%> (ø)

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