Add POD overview on hover in code and autocomplete

[antoinemarechal]

Having the documentation at hand would be a nice addition. This should be achievable with the perldoc utility using variations of perldoc -T -MPod::Simple::HTML <other arguments>. The formatter would default to Pod::Simple::HTML but would be customizable in the extension settings.

This feature could cover the perlfunc, perlapifunc, perlvar (respectively with the arguments -f, -a and -v) and modules.

Regarding the hover/autocomplete of module subroutines/constants/variables, it could just display the documentation of the whole module. As this level of granularity is not ideal, the process could be made a little more smart about those symbols. The idea would be to search after =headX or =item occurences followed by the symbol name in the POD and if results are found, the documentation would be scrolled to the first occurence. As the results might not be perfect, this could be an optional feature to toggle in the settings.