Add a specification for the JS API importer's this context (#3052)

This will allow me to write a fast-track proposal to add metadata
about whether a load is coming from `@import` or not, to address
webpack-contrib/sass-loader#905.

spec/js-api.md

@@ -0,0 +1,47 @@
+# JavaScript API
+
+Sass implementations that are available for use via JavaScript must expose the
+following JavaScript API. As with the rest of this specification, they must not
+add custom extensions that aren't shared across all implementations.
+
+> Having a shared, consistent API makes it easy for users to move between Sass
+> implementations with minimal disruption, and for build system plugins to
+> seamlessly work with multiple implementations.
+
+As with other sections of this specification, the specification of the JS API is
+incomplete, and is added to *lazily*. This means that portions of the spec are
+only written when they're necessary as background for new API proposals.
+
+## Table of Contents
+
+* [Importers](#importers)
+  * [`this` Context](#this-context)
+
+## Importers
+
+### `this` Context
+
+When running an importer callback, JavaScript's `this` must refer to an object
+with an `options` field. This field's value must be an object with the following
+fields:
+
+* `file`: The `file` option passed to the `render()` call.
+* `data`: The `data` option passed to the `render()` call.
+* `includePaths`: A string that contains the current working directory followed
+  by strings passed in the `includePaths` option, separated by `";"` on Windows
+  and `":"` elsewhere.
+* `precision`: The number 10.
+* `style`: An integer. The specific semantics of this are left up to the
+  implementation. (The reference implementation always returns 1.)
+* `indentType`: The number 1 if the `indentType` option was `tab`. The number 0
+  otherwise.
+* `indentWidth`: An integer indicating the number of spaces or tabs emitted by
+  the compiler for each level of indentation.
+* `linefeed`: A string indicating the linefeed character or character sequence
+  emitted by the compiler at the end of each line.
+* `result`: An object with a `stats` field, whose value is an object with the
+  following fields:
+  * `start`: The number of milliseconds since the Unix epoch (1 January 1970
+    00:00:00 UT) at the point at which the user called `render()`.
+  * `entry`: The `file` option passed to the `render()` call, or the string
+    `"data"` if no file was passed.