Modifying default variables and config

Renamed variables for defaults:
- flatpak_method -> default_flatpak_method: system
- defaults.paths.prefix -> default_install_prefix: /usr/local
New variables:
- default_install_method: package

tasks/helpers/npm.yml

@@ -0,0 +1,8 @@
+# vim: set filetype=yaml.ansible :
+---
+- name: Install npm {{ pkg }}
+  become: true
+  community.general.npm:
+    global: true
+    name: "{{ pkg }}"
+    state: present