Create Cursor class

[jerch]

Currently our cursor handling is scattered over several classes:

• x/y values live in Buffer
• positioning / movements happen in InputHandler
• position is requested at various places with repeated sanity checks like if (buffer.x >= terminal.cols)

Proposal:

Lets unify the cursor handling with an explicit class holding the position and doing proper range checks during re-positioning.

Stub:

interface ICursor {
  readonly x: number;
  readonly y: number;
  readonly cols: number;  // maybe private or directly inherited from Buffer
  readonly rows: number;  // maybe private or directly inherited from Buffer
  move(x: number, y: number): void;  // relative movements
  set(x: number, y: number): void;   // absolute positions
}

For set and move the class can automatically enforce to stay within cols/rows limits. These limits could auto-update by terminal resize and such.

[mofux]

Will a cursor instance then become part of a buffer instance? E.g. term.buffer.cursor.move(1,1)?

[Tyriar]

@mofux I don't think the API surface will change, we want people to adjust the cursor position using term.write instead of directly moving the cursor.

[jerch]

note to myself:
Seems relative cursor positioning always has to respect the scroll borders and should not pass the area even if DECOM is disabled. (See test file t0075-DECSTBM_CUU_CUD.in and workaround in
0029853)
Might also apply to left/right borders once supported.

[Tyriar]

Closing as it probably won't end up happening anyway, cursor is currently managed in buffer.ts and doesn't seem too bad atm.