feat: add CustomSyntax

Author: pengzhanbo

src/config/customSyntax.d.ts

@@ -0,0 +1,17 @@
+import type { LiteralUnion } from '../utils'
+
+export interface CustomSyntax {}
+
+export type KnownCustomSyntax = LiteralUnion<
+  | 'postcss-angular'
+  | 'postcss-html'
+  | 'postcss-less'
+  | 'postcss-linaria'
+  | 'postcss-lit'
+  | 'postcss-markdown'
+  | 'postcss-sass'
+  | 'postcss-scss'
+  | 'postcss-styled-components'
+  | 'postcss-styled-syntax'
+  | keyof CustomSyntax
+>