Add possibility to disable all welcome screens. Fixes #1049.

[rickardp]

Description

Issue #1049 is about disabling all welcome screens altogether, including both the first-launch and the major-update. These become very annoying when used inside development containers and/or GitHub codespaces as this type of state is typically lost several times a day.

I made the choice to disable all welcome screens with one setting (leaving the "what's new" setting unchanged). The reasoning is that, for my use case above, I would prefer to just disable all the "advertising", current and future, with one setting that I could add to the devcontainer.json of my repos.

Checklist

• I have followed the guidelines in the Contributing document
• My changes follow the coding style of this project
• My changes build without any errors or warnings
• My changes have been formatted and linted
• My changes include any required corresponding changes to the documentation
• My changes have been rebased and squashed to the minimal number (typically 1) of relevant commits
• My changes have a descriptive commit message with a short title, including a Fixes $XXX - or Closes #XXX - prefix to auto-close the issue that your PR addresses

[eamodio]

Thank you for your contribution!

[eamodio]

FYI, I changed the setting to be gitlens.showWelcomeOnInstall and only affect the welcome experience on the first install. If you also want to disable the What's New notification you will also need to change the existing gitlens.showWhatsNewAfterUpgrades setting.