Let `some-sass-language-server` handle all features for CSS and SCSS

[wkillerud]

The "contract" for some-sass-language-server right now is that it should behave well (ish) with vscode-css-language-server running and providing features for SCSS in the same editor (basically mirror what happens in VS Code out of the box). Unfortunately that's not always the case (#233), and in a way it makes it more complicated to configure editors that don't ship with vscode-css-language-server built in.

We want to change the language server so it handles all features for CSS, SCSS and Sass indented by default (provided the editor is configured to run some-sass-language-server for them). This is considered a breaking change, meaning a new semver major version.

Editors that don't ship with vscode-css-language-server will have an easier time configuring SCSS, Sass indented, and the option of using this server also for CSS.

For editors that do ship with vscode-css-language-server turned on by default for SCSS (VS Code, VSCodium, perhaps others):

• the out-of-the-box experience should be similar to the current behavior (features handled by vscode-css-language-server should be turned off in some-sass by default).
• the user should be able to turn off vscode-css-language-server and configure some-sass to handle everything.

[wkillerud]

Since this will be a breaking change I want to take some time to consider other breaking changes, like how the settings are named and organized, and perhaps drop the development export from the published module.

If you have input on things that would make it easier to use some-sass-language-server in your editor, feel free to comment 🙌