Readonly helps out with TS's type guards and is also a good best practice. Specifically, readonly strings/numbers have their literal type instead of a generic `string` | `number` type, which can help catch dead code and some common programming mistakes

- Also switches a few places that re-implemented Disposable with calls to `_register` instead

Prefer using these predicates as they are shorter and make the code easier to understand

- Import aliasing symbol in file
- Missing function calls in conditional
- Missing return statement

Use an empty string instead of null for reseting styles

Fixes cases where styles value can no longer be null

For #81574

Fixes #81887

Related xtermjs/xterm.js#291

and handle flipped highlight start and end

Fixes #62211

Fixes #81911

Fixes #81949

Fixes https://github.com/microsoft/vscode/issues/73580

#81574

Fixes https://github.com/microsoft/vscode/issues/78817

related to #78168

This reverts commit 4fe8b829.