Join Lines (cmd+j) should use a `"when"` clause.

[alexdima]

Extracted from microsoft/vscode#97906

vscode-atom-keybindings/package.json

Lines 520 to 526 in 2b7cd33

	{
	"mac": "cmd+j",
	"linux": "ctrl+j",
	"win": "ctrl+j",
	"key": "ctrl+j",
	"command": "editor.action.joinLines"
	},

The "when" clause should be "editorTextFocus && !editorReadonly".