Fix TextEdit mouse selection and scroll cancel

[kitbdev]

• fixes Code selection does not allow scrolling while moving the mouse #91742
• fixes GDScript scrolling while selecting text #73290 (same)
• fixes (partially) Text is selected unexpectedly in script editor when left clicking after opening a context menu #95471

adjust_viewport_to_caret now only cancels scrolling if it moves the viewport. Moved the scroll cancel into set_line_as_*_visible.

Made sure mouse selection stops when: right clicking, IME updates, adding caret above/below, selecting word, adding selection for next occurrence, and skip selection for occurrence.

Disabled the click_select_held timer when selection mode is shift.

[kitbdev]

Rebased.

• Text is selected unexpectedly in script editor when left clicking after opening a context menu #95471 is only incidentally fixed.

The purpose of adding set_selection_mode(SELECTION_MODE_NONE); to when the context menu opens is so that if the user is selecting text and holding down LMB and clicks RMB then text stops getting selected.
What happens is that, in multi-window mode, when the context menu popup is closed by clicking off of it, the TextEdit receives an InputEventMouseMotion with mouse flag set without a corresponding pressed InputEventMouseButton beforehand. When this happens, it continues with the last mode it was in, which is now corrected to be none.
However, The issue can still happen if a different popup was opened, like the File menu or a non TextEdit context menu.