Add pure css parser (#7933)

changelog_unreleased/css/pr-7933.md

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+#### [BREAKING] Add pure css parser ([#7933](https://github.com/prettier/prettier/pull/7933) by [@fisker](https://github.com/fisker))
+
+Previously when `--parser=css` passed, we try `postcss-scss`/`postcss-less` to parse content, this cause confusion, and hard to check syntax error. Now `--parser=css` only works on css syntax.
+
+_If you're setting `parser="css"` for your `.less`/`.scss` file, you'll need update it to correct parser, or let prettier decide._
+
+<!-- prettier-ignore -->
+```less
+/* Input */
+/* Less Syntax with `--parser=css` */
+a {.bordered();}
+
+/* Prettier stable */
+/* Less Syntax with `--parser=css` */
+a {
+  .bordered();
+}
+
+/* Prettier master */
+SyntaxError: (postcss) CssSyntaxError Unknown word (2:4)
+  1 | /* Less Syntax with `--parser=css` */
+> 2 | a {.bordered();}
+```
+
+<!-- prettier-ignore -->
+```scss
+/* Input */
+/* Scss Syntax with `--parser=css` */
+::before {content: #{$foo}}
+
+/* Prettier stable */
+/* Scss Syntax with `--parser=css` */
+::before {
+  content: #{$foo};
+}
+
+/* Prettier master */
+SyntaxError: (postcss) CssSyntaxError Unknown word (2:22)
+  1 | /* Scss Syntax with `--parser=css` */
+> 2 | ::before {content: #{$foo}}
+```