Pass sass options to plugin

[drwpow]

Changes

This doesn’t change how the plugin operates at all; just allows the plugin to accept flags Sass would be able to accept normally:

• style
• sourceMap
• sourceMapUrls
• embedSources
• embedSourceMap
• charset
• update

While some of these options may be used more than others, it still helps our plugin to feel less like a restriction and more like a boost for Sass users.

Testing

Tests were added!

Docs

Plugin docs updated.

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[FredKSchott]

+1 on the concept here, but also this is so relevant to the work we just did with Svelte. The lesson from that was "pass compiler options as a separate config, and not the top-level config". You can see them grappling with this decision 2 years later, here: sveltejs/svelte#1101 (comment)

Given that we just made that change for Svelte, lets follow the same pattern here. Instead of supporting all of these on the top -level object and then picking them out, lets instead support them on a single compilerOptions option that we can serialize and pass directly to Sass.