Fix block cursor to render text with cursorAccent color

[ncoop57]

Problem

The cursorAccent theme property was defined in xtermjs' ITheme but never actually used. Block cursors rendered as opaque rectangles that completely cover the character underneath, making it invisible.

Solution

• Added optional colorOverride parameter to renderCellText()
• After drawing the block cursor, re-render the cell's text using cursorAccent as the color override

This is a minimal change that reuses the existing text rendering logic, ensuring proper handling of font styles, graphemes, and wide characters.

PR created with the assistance of SolveIT, manually reviewed, iterated, and tested personally.