Add the pageref element

[LeSuisse]

This instruction is useful if you want to get the number of the page containing a specific bookmark.

[codecov-commenter]

Codecov Report

Merging #1074 (3f257bf) into master (1401327) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1074   +/-   ##
=======================================
  Coverage   99.29%   99.30%           
=======================================
  Files         311      313    +2     
  Lines        3557     3578   +21     
  Branches      379      381    +2     
=======================================
+ Hits         3532     3553   +21     
  Misses         24       24           
  Partials        1        1           

Impacted Files	Coverage Δ
src/file/paragraph/links/index.ts	100.00% <100.00%> (ø)
.../file/paragraph/links/pageref-field-instruction.ts	100.00% <100.00%> (ø)
src/file/paragraph/links/pageref.ts	100.00% <100.00%> (ø)

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[dolanmiu]

Looks good!