Grammar is slow on a line with a very long string literal (with embedded json)

## Details

What editor are you seeing the problem in? (e.g. Atom, Visual Studio Code, etc.)
VS Code

What version of the editor are you using?
1.66 insiders and 1.65 stable.

What color theme are you using?
N/A

## Repro

This file has the issue - [example.txt](https://github.com/microsoft/vscode/files/8332370/example.txt) - although in the original it contained a bearer token, so I couldn't include verbatin - all chars replaced with x.

_Originally reported by @kierenj in https://github.com/microsoft/vscode/issues/145876._