add na.rm argument to first/last and gfirst/glast

[ben-schwen]

Closes #4446
Closes #4239

Adds an na.rm argument with default na.rm=FALSE to first()/last() and gfirst()/glast().

Here should be noted that gfirst() and glast() return the first resp. last non NA value if such a value exists. If such a value does not exist gfirst()/glast() still return NA similar to gmedian(), gvar() and gsd().

[codecov]

Codecov Report

Merging #4730 into master will decrease coverage by 0.25%.
The diff coverage is 67.24%.

@@            Coverage Diff             @@
##           master    #4730      +/-   ##
==========================================
- Coverage   99.44%   99.18%   -0.26%     
==========================================
  Files          73       73              
  Lines       14539    14649     +110     
==========================================
+ Hits        14458    14530      +72     
- Misses         81      119      +38     

Impacted Files	Coverage Δ
src/gsumm.c	96.50% <63.26%> (-3.12%)	⬇️
R/last.R	96.87% <87.50%> (-3.13%)	⬇️
R/data.table.R	100.00% <100.00%> (ø)

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[jangorecki]

last comment about styling, other than that it looks good

[ben-schwen]

Closing here since incompatible with merged #5109