TextField supports input masks

TextField should support input masks: a pattern that constrains what the user can type and shows the fixed characters for them. For example, an InputMask of ##/##/#### for a UK date shows __/__/____, accepts digits only, and auto-inserts the slashes, so typing 29092026 yields 29/09/2026. This removes the convoluted TextChange-plus-string-comparison workaround users currently need, which cannot handle caret position, selection, and paste correctly. Both desktop and web TextFields get the feature.