feat: add submit semantics, paste support to MultiLineInput, and Mult…

[iyadi]

…ilinePromptDemo app and unicode support to the input line

[codecov]

Codecov Report

❌ Patch coverage is 87.91209% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...een/src/main/scala/termflow/tui/AnsiRenderer.scala	69.23%	4 Missing ⚠️
...ain/scala/termflow/tui/ConsoleKeyPressSource.scala	71.42%	4 Missing ⚠️
...rc/main/scala/termflow/tui/widgets/TextField.scala	93.93%	2 Missing ⚠️
...mflow-app/src/main/scala/termflow/tui/Keymap.scala	0.00%	1 Missing ⚠️

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