[PR #6741/568814fc backport][stable-7] New module: pnpm package manager (#7225)

New module: pnpm package manager (#6741)

* (feat) New module pnpm added

A new module for pnpm is added. Necessary entries were written to
BOTMETA.yml.

* (feat) Basic tests added

* (feat) reduced nesting of ifs

* (fix) trying to fix up CI pipelines

* (fix) incorrect indentation of alias fixed

* (feat) fixed further indentations

* Apply suggestions from code review

Co-authored-by: Felix Fontein <felix@fontein.de>

* (fix) various linting and CI errors fixed/ignored

* (feat) reduced restriction, new install method

Some restrictions on OS are reduced. New installation method, similar to
the official installation method, is provided.

* (fix) ignoring CentOS 6 for CI.

* retrigger checks

---------

Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 568814fc3e)

Co-authored-by: Aritra Sen <125266845+aretrosen@users.noreply.github.com>

.github/BOTMETA.yml

@@ -980,6 +980,9 @@ files:
     maintainers: $team_solaris dermute
   $modules/pmem.py:
     maintainers: mizumm
+  $modules/pnpm.py:
+    ignore: chrishoffman
+    maintainers: aretrosen
   $modules/portage.py:
     ignore: sayap
     labels: portage