Expose specific option change listener on options service

[Tyriar]

Currently we only have the onOptionChange event:

xterm.js/src/common/services/OptionsService.ts

Line 102 in a22ad76

this._onOptionChange.fire(propName);

Which leads to checking the options on the other side, this is especially ugly in the larger files:

xterm.js/src/browser/Terminal.ts

Lines 273 to 317 in a22ad76

	// When the font changes the size of the cells may change which requires a renderer clear
	this._renderService?.clear();
	this._charSizeService?.measure();
	break;
	case 'cursorBlink':
	case 'cursorStyle':
	// The DOM renderer needs a row refresh to update the cursor styles
	this.refresh(this.buffer.y, this.buffer.y);
	break;
	case 'customGlyphs':
	case 'drawBoldTextInBrightColors':
	case 'letterSpacing':
	case 'lineHeight':
	case 'fontWeight':
	case 'fontWeightBold':
	case 'minimumContrastRatio':
	// When the font changes the size of the cells may change which requires a renderer clear
	if (this._renderService) {
	this._renderService.clear();
	this._renderService.handleResize(this.cols, this.rows);
	this.refresh(0, this.rows - 1);
	}
	break;
	case 'scrollback':
	this.viewport?.syncScrollArea();
	break;
	case 'screenReaderMode':
	if (this.optionsService.rawOptions.screenReaderMode) {
	if (!this._accessibilityManager && this._renderService) {
	this._accessibilityManager = new AccessibilityManager(this, this._renderService);
	}
	} else {
	this._accessibilityManager?.dispose();
	this._accessibilityManager = undefined;
	}
	break;
	case 'tabStopWidth': this.buffers.setupTabStops(); break;
	case 'theme':
	this._setTheme(this.optionsService.rawOptions.theme);
	break;
	}
	}
	/**
	* Binds the desired focus behavior on a given terminal object.

This sort of thing would encapsulate this repetitive pattern inside options service:

IOptionsService.onSpecificOptionChange(key: 'x' | 'y' | ..., listener: () => void