Make .I available in i

[jan-glx]

From the documentation:

.SD, .BY, .N, .I, and .GRP are read-only symbols for use in j. .N can be used in i as well.

But like .N, .I could be made available in i as well. (As in j?,) it would be mostly syntactic sugar for seq_len(.N).
For example,

> DT <- data.table::data.table(letters[1:5])
> DT
   V1
1:  a
2:  b
3:  c
4:  d
5:  e
> DT[seq_len(.N) < 3]
   V1
1:  a
2:  b
> DT[.I < 3]
Empty data.table (0 rows and 1 cols): V1

If implemented, DT[.I < 3] would return

   V1
1:  a
2:  b

as well.

I've searched extensively and unsuccessfully for an existing issue but searching for two symbol words is not so easy so I might have missed it; In that case: sorry for the noise.

> sessioninfo::session_info()
─ Session info ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 setting  value                       
 version  R version 4.0.0 (2020-04-24)
 os       CentOS Linux 7 (Core)       
 system   x86_64, linux-gnu           
 ui       RStudio                     
 language (EN)                        
 collate  en_US.UTF-8                 
 ctype    en_US.UTF-8                 
 tz       Europe/Berlin               
 date     2020-09-06                  

─ Packages ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 package        * version    date       lib source                            
 data.table     * 1.12.8     2019-12-09 [1] CRAN (R 4.0.0)        

[ColeMiller1]

#4585 seems unrelated, but includes a way to evaluate i with constants. Currently .N is in there but there is discussion regarding .SD. It would be trivial to have .I. See this discussion regarding .SD in i

#4585 (comment)