feat: add p/moul/helplink

[moul]

This PR aimed to promote the use of a p/ library for managing special help links from contracts.

It also provided an opportunity for me to realize that our discussion about changing the $ symbol would require some parsing and detection from the gnoweb perspective. If we want a simple library like this one, the goal should be to ideally craft a link to the current package without specifying the realm path. Relative URLs worked well with ?, but they won't function with $.

As an alternative, we can have this package look for std.PrevRealm().PkgAddr if it is not specified.

cc @jeronimoalbi @thehowl @leohhhn

Related with #2602
Related with #2876

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.96%. Comparing base (8a62a28) to head (c214869).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2887      +/-   ##
==========================================
+ Coverage   60.95%   60.96%   +0.01%     
==========================================
  Files         564      564              
  Lines       75273    75273              
==========================================
+ Hits        45884    45892       +8     
+ Misses      26019    26011       -8     
  Partials     3370     3370              

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	15.31% <ø> (+0.85%)	⬆️
gno.land	67.92% <ø> (ø)
gnovm	65.77% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	19.88% <ø> (ø)
tm2	62.12% <ø> (+0.04%)	⬆️

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