Add support for parenthesis in "when" conditions

• 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

[Spongman]

at the very least the Keyboard Shortcuts UI should reject inputs that contain parentheses (if they’re not supported currently) instead of garbling the user input resulting in non-functioning expressions and no undo.

[a-stewart]

I’ve created a draft cl / POC for how parentheses could be handled in a way that is unlikely to cause issue with existing parentheses in values and regex logic.

Still needs work, in particular regarding any conflicts with existing when clauses using brackets in regex, but it might be useful to open a discussion