Make cursor blinking configurable in TerminalView

[dianedef]

Summary

• add a nullable cursorBlink rendering override without mutating terminal state
• expose cursorBlinkInterval and a nullable cursorBlinkTimeout
• preserve existing behavior by default (terminal-controlled blinking, 750 ms interval, 5 s timeout)
• allow uninterrupted blinking with cursorBlinkTimeout: null

Motivation

Embedding applications may need a stable cursor preference independent of DECSCUSR sequences, including disabling motion for accessibility or keeping a focused cursor blinking continuously. The nullable override keeps application-controlled cursor behavior unchanged unless the host explicitly opts in.

Tests

Widget tests cover:

• unchanged default timing and timeout
• custom interval and timeout
• cursorBlink: null following terminal state
• cursorBlink: true overriding a non-blinking terminal
• cursorBlink: false overriding a blinking terminal
• cursorBlinkTimeout: null continuing beyond five seconds

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