Adding the keymap keyword to package.json

[a-stewart]

The keybindings created by Microsoft seem to all use the "keymap" keyword, adding this keyword here:

https://github.com/microsoft/vscode-sublime-keybindings/blob/main/package.json https://github.com/microsoft/vscode-atom-keybindings/blob/main/package.json

[kasecato]

This was a pretty important addition. Thanks.