Calculate mouse coordinates using correct char dimensions

Previously the Mouse class was using raw CSS pixels, we need CSS pixels based
on the amount of space the cells take up on the canvas

See Microsoft/vscode#35114

src/Interfaces.ts

@@ -47,6 +47,7 @@ export interface ITerminal extends ILinkifierAccessor, IBufferAccessor, IElement
   options: ITerminalOptions;
   buffers: IBufferSet;
   isFocused: boolean;
+  mouseHelper: IMouseHelper;
 
   /**
    * Emit the 'data' event and populate the given data.
@@ -174,6 +175,11 @@ export interface IBufferSet {
   activateAltBuffer(): void;
 }
 
+export interface IMouseHelper {
+  getCoords(event: {pageX: number, pageY: number}, element: HTMLElement, charMeasure: ICharMeasure, lineHeight: number, colCount: number, rowCount: number, isSelection?: boolean): [number, number];
+  getRawByteCoords(event: MouseEvent, element: HTMLElement, charMeasure: ICharMeasure, lineHeight: number, colCount: number, rowCount: number): { x: number, y: number };
+}
+
 export interface IViewport {
   syncScrollArea(): void;
   onWheel(ev: WheelEvent): void;