Write a blog post for the new JS API RC (sass#600)

Co-authored-by: Awjin Ahn <awjin@google.com>

source/blog/031-new-js-api-release-candidate.html.md

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+---
+title: "New JS API Release Candidate is Live"
+author: Natalie Weizenbaum
+date: 2021-11-20 16:15 PST
+---
+
+The new JavaScript API that we [announced a few months ago] is now fully
+implemented in Dart Sass and ready for you to try! The new API is designed to be
+more idiomatic, performant, and usable than the old one, and we hope it'll be
+adopted swiftly by tooling packages.
+
+Because this is such a substantial addition, we want to give users a chance to
+kick the tires a bit before we set it in stone, so we've released it as a release
+candidate in Dart Sass 1.45.0-rc.1. Download it, try it out, and let us know
+what you think by [filing issues] or [sending us a tweet]. Unless major changes
+are necessary, we plan to make a stable release some time next week.
+
+[announced a few months ago]: https://sass-lang.com/blog/request-for-comments-new-js-api
+[filing issues]: https://github.com/sass/sass/issues/new
+[sending us a tweet]: https://twitter.com/SassCSS
+
+## How to use it
+
+The new API comes with four new entrypoint functions: `compile()` and
+`compileAsync()` take Sass file paths and return the result of compiling them to
+CSS, while `compileString()` and `compileStringAsync()` take a
+string of Sass source and compile it to CSS. Unlike the old API, the async
+functions all return `Promise`s. As with the old API, the synchronous functions
+are substantially faster than their async counterparts, so we recommend using
+them if at all possible.
+
+```js
+const sass = require('sass');
+
+const result = sass.compileString(`
+h1 {
+  font-size: 40px;
+  code {
+    font-face: Roboto Mono;
+  }
+}`);
+console.log(result.css);
+```
+
+Check out [the API docs] for more details on the API, including the brand-new
+importer and custom function APIs.
+
+[the API docs]: /documentation/js-api
+
+## What about the old API?
+
+Once the new API has a stable release, we'll officially consider the old API to
+be deprecated. Since it's still widely-used, we'll continue to maintain it for a
+good long while going forward. Expect it to start printing a deprecation warning
+in a year or so, and to be disabled for good once we release Dart Sass 2.0.0.