Add support for project.viewport

[Mortaro]

Project should have a viewport key that is used to generate the template

you should be able to configure the viewport by both using

project.viewport = "width=device-width, initial-scale=1, shrink-to-fit=no";

or

NULLSTACK_PROJECT_VIEWPORT="width=device-width, initial-scale=1, shrink-to-fit=no"

The default value for viewport should be

width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no viewport-fit=cover

relevant files

• https://github.com/nullstack/nullstack/blob/master/server/template.js
• https://github.com/nullstack/nullstack/blob/master/server/project.js
• https://github.com/nullstack/nullstack/blob/master/tests/src/ContextProject.njs