Add mmHg to pressure options in weather applet

Summary:
Add millimeters of mercury (mmHg) to pressure options in weather applet, which is a common pressure unit in some countries

FEATURE: 401734
FIXED-IN: 5.18.0

Test Plan: You can select 'mmHg' as the pressure unit and it displays correctly in the widget

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24952