expose "system" color mode

[teidesu]

Is your feature request related to a problem? Please describe.
currently it's not possible to easily implement checkmarks in a color mode switcher like this:

because colorMode accessor only returns a dark/light value, and not whether it's currently linked to system color scheme. in other words, there isn't an easy way to know if setColorMode('system') was called

Describe the solution you'd like
a relatively easy way to implement this would be to expose isSystemMode() accessor in the context.
alternatively, colorMode() could return 'dark' | 'light' | 'system', but that would be a breaking change

Describe alternatives you've considered
currently we have to do crutches involving manually reading data from localStorageManager to implement this, which is definitely not ideal