Add deprecation tests for custom-ident functions and mixins (#1982)

See #1962

js-api-spec/deprecations.test.ts

@@ -38,6 +38,7 @@ const activeDeprecations: {[key in keyof Deprecations]?: string} = {
   'null-alpha': '1.62.3',
   'abs-percent': '1.65.0',
   'fs-importer-cwd': '1.73.0',
+  'css-function-mixin': '1.76.0',
 };
 
 /**

spec/directives/function.hrx

@@ -0,0 +1,32 @@
+<===> escaped/input.scss
+// Function names can be defined and referred to using escapes, which are
+// normalized.
+@function f\6Fo-bar() {@return 1}
+
+a {b: foo-b\61r()}
+
+<===> escaped/output.css
+a {
+  b: 1;
+}
+
+<===>
+================================================================================
+<===> custom_ident_name/input.scss
+@function --a() {@return 1}
+b {c: --a()}
+
+<===> custom_ident_name/output.css
+b {
+  c: 1;
+}
+
+<===> custom_ident_name/warning
+DEPRECATION WARNING on line 1, column 11 of input.scss: 
+Sass @function names beginning with -- are deprecated for forward-compatibility with plain CSS mixins.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+  ,
+1 | @function --a() {@return 1}
+  |           ^^^
+  '

spec/directives/mixin.hrx

@@ -0,0 +1,18 @@
+<===> custom_ident_name/input.scss
+@mixin --a {b: c}
+d {@include --a}
+
+<===> custom_ident_name/output.css
+d {
+  b: c;
+}
+
+<===> custom_ident_name/warning
+DEPRECATION WARNING on line 1, column 8 of input.scss: 
+Sass @mixin names beginning with -- are deprecated for forward-compatibility with plain CSS mixins.
+
+For details, see https://sass-lang.com/d/css-function-mixin
+  ,
+1 | @mixin --a {b: c}
+  |        ^^^
+  '