Allow custom includePaths for sass

[lixiaoyan]

Allow developers to customize includePaths for sass-loader by specify environment variable SASS_INCLUDEPATHS, e.g. SASS_INCLUDEPATHS="./node_modules".

See also: material-components/material-components-web-react#109.

Fixes #4494, #4651.

[MarkLSky]

SASS already has support for the SASS_PATH env variable - http://sass-lang.com/documentation/Sass.html

How about just allowing this instead instead of introducting new config?

I was able to get this working just by adding this one line to env.js:

...
PUBLIC_URL: publicUrl,
SASS_PATH: process.env.SASS_PATH

[willbamford]

Adding the following to .env at root of project worked for me (basically similar/same approach as @MarkLSky has suggested):

SASS_PATH=node_modules

[jayantbh]

Same as #5599 (comment),
For multiple directories, you can do this:

SASS_PATH=node_modules:src:some_directory