Dark mode editor support.

[Lego6245]

This is part RFC, part code review, for dark mode support in rooster js. For sample site implementation, please see forthcoming PR which will have this as a dependent.

Philosophy:
Editor, on create, will accept an additional parameter for dark mode support. If provided (either a boolean or a DarkModeOptions object), the editor will be in dark mode.

Dark mode is enabled by having two data propertie, ogsc and ogsb. ogsc represents the color for light mode, ogsb represents the background color for light mode. Conversion is currently one way.

Editor will expose two new APIs:

• isDarkMode -> returns a boolean that will indicate if the editor is in dark mode or not
• getDarkModeOptions -> returns the DarkModeOptions object.

Editor's API will change:

• getContent will take an additional parameter. It will, by default, always return the "light" mode of content, unless passed a boolean to get the dark mode content, if it exists. Otherwise, it's a no-op if in light mode.
  There's a new helper method to convert from dark mode to light mode.

Editor Core will have two new properties, isDarkMode and darkModeOptions to facilitate the above. getDefaultFormat will rectify the dark mode options instead of the other defaults if dark mode is enabled.

DefaultFormat has been expanded to provide the ability to set default ogsc and ogsb properties. Dependent files (applyFormat) have been updated to allow for this as well.

setTextColor and setBackgroundColor both take in ogsc and ogsb parameters now.

Paste.ts now applies a transform to pasted content if dark mode is enabled. We provide our own default (strip color) but allow a user to define a behavior through onExternalContentTransform.

Soliciting feedback on the entire project, but I'll ask questions inline.

[Adjective-Object]

for setBackgroundColor, use of ogsb/c in code, misleading parameter names.

[Adjective-Object]

Looks like you missed some changes in the refactor because of casting.

lgtm otherwise! 👍

[JiuqingSong]

nit: setTextColor(editor, defaultFormat.textColors || defaultFormat.textColor);
(current way is also ok to me)

[adriantran]

curious to know if this works with copy-paste scenarios

[Lego6245]

Pasting content in will be supported (see the changes to Paste.ts)

Copying from editor into another surface currently isn't normalized. I'd do that outside this PR just because of its current size.