Add support for parenthesis in "when" conditions

[Veetaha]

• VSCode Version: 1.42.0
• OS Version: Ubuntu 18.04.3

The when expression that contains parentheses which we set for a keybinding defined in the contribution point of our extension results into a corrupted (invalidly transformed expression in user's keybinding settings), e.g. the following declaration:

{
    "command": "extension.helloWorld",
    "key": "enter",
    "when": "editorLangId == rust && !suggestWidgetVisible && (!vim.active || vim.mode == 'Insert')"
}

produces the following invalid when expression:

Steps to Reproduce:

1. Go to the following repo with the reproduction
2. Follow the steps in README.md

Does this issue occur when all extensions are disabled?: Yes

[yasintz]

It should be able to properly handle something like
( (a&&(b|| c) && d) ) && !e && false && true || !(f != g) && (h == i).

[sandy081]

Keybindings editor is rendering the text generated by when context serialize method.