Dev tokens trim

[koheiw]

For #2402, but tokens_trim() and dfm_trim() do not print verbose messages, because they should be added later in #2396.

I think the sparsity argument should be deprecated as it is the same as min_docfreq where docfreq_type = "prop".

[codecov]

Codecov Report

Attention: Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.26%. Comparing base (2d2f5dd) to head (f54d3b2).
Report is 14 commits behind head on master.

Files with missing lines	Patch %	Lines
R/dfm_trim.R	96.87%	1 Missing ⚠️
R/tokens_trim.R	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2419      +/-   ##
==========================================
+ Coverage   96.22%   96.26%   +0.03%     
==========================================
  Files          94       95       +1     
  Lines        5909     5918       +9     
==========================================
+ Hits         5686     5697      +11     
+ Misses        223      221       -2     

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