feat(@angular/cli): support includePaths for less (closes #7083)

[JPeer264]

This should solve #7083.

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[JPeer264]

I signed it!

[googlebot]

CLAs look good, thanks!

[filipesilva]

Clean change and test, LGTM!

[PikaChokeMe]

Is this supposed to be working right this second?
I've tried adding

"styles": [
                "styles/colors.less",
                "styles/mixins.less",
                "../node_modules/quill/dist/quill.core.css",
                "../node_modules/quill/dist/quill.snow.css"
            ],
            "stylePreprocessorOptions": {
                "includePaths": [
                    "styles"
                ]
            },

To my project as well as having

| my-angular-app
 \ src
   \styles
    | mixins.less
    | colors.less

Where I have
@tablet-view: 1023px;

But after trying to use @tablet-view in one of my components.less files I end up with

ERROR in ./src/app/modules/my-module/components/some-page/my-page.component.less
Module build failed: 


        .respond-max(@tablet-view - 1; { order: 3; flex: 1 0 100%; });
                   ^
Variable @tablet-view is undefined
      in C:\Users\MyUser\Source\my-angular-app\src\app\modules\my-module\components\some-page\my-page.component.less (line 39, column 21)

Any advice or tips on why it might not be working?

[JPeer264]

@BustyLoli-Chan have you put a @import 'filename' at the top of your file?

I assume that you have defined everything in mixins.less, then it should look like following:

@import 'mixins';

.respond-max(@tablet-view - 1; { order: 3; flex: 1 0 100%; });

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