Minimal IE Runtime

[nhooyr]

See #445

Summary

Adds a very minimal runtime for IE. Not the most robust but should work well for our purposes.

See #445

Test plan

Not really sure yet.

[nhooyr]

The way it work is that when a css variable needs to be modified, instead of just adding an inline style, the IE code will find the rule in the stylesheet that uses the css variable on the class, replace the variable with the value and add this new rule to a dynamically injected stylesheet.

[nhooyr]

Not sure if the rule cache here is necessary. For the rule on every dynamic CSS variable, is the selector just the class?

[pbitkowski]

Thanks for your contribution! Is this possible to extract this logic to another library, to make this opt-in feature? I know it's tiny, but we don't to do it by default to keep our core small and happy.

[dreyks]

@pbitkowski how do you feel about making it a separate library indeed but scoping it under callstack? that way the core could still be kept small but the library won't get lost in the future?

[nhooyr]

Don't have the time to work on this anymore. Someone else feel free to continue.