LibGUI/TextEditor: Shift+Tab unindents line

[theeyeofcthulhu]

Unindent the current line if Shift+Tab is pressed and selection is made.

[BuggieBot]

Hello!

One or more of the commit messages in this PR do not match the SerenityOS code submission policy, please check the lint_commits CI job for more details on which commits were flagged and why. Please do not close this PR and open another, instead modify your commit message(s) with git commit --amend and force push those changes to update this PR.

[LucasChollet]

It is required to use a capitalized letter after the colon on the commit message. Please also squash both commits, the first one will otherwise add dead code.