Add shortcut Shift + F3 to search pervious in the built-in docs

When using the built-in docs, Godot would not support the shortcut "Shift + F3"
to search for the previous occurrence of the search entry text, thus causing an
inconsistent behaviour when using shortcuts in the "ScriptEditor" compared to
using them in the "ScriptTextEditor".

The previous parameter of the function "EditorHelp::_search()" in the class
"editor_help" seems to be unused, thus replaced with a bool representing to
search for previous search entry text or not. By adding the shortcut to
Godot's "ScriptEditor", this commit now improves Godot's consistensy when
using shortcuts.

Fixes #31147.

Co-Authored-By: Oscar Ferm <oscfer-6@student.ltu.se>

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