refactor: extract shared utilities (isMarkdownFile, isPlainObject, resolveSymlink) (#33)

src/shared/snake-case.ts

@@ -1,3 +1,5 @@
+import { isPlainObject } from "./deep-merge"
+
 export function camelToSnake(str: string): string {
   return str.replace(/[A-Z]/g, (letter) => `_${letter.toLowerCase()}`)
 }
@@ -6,10 +8,6 @@ export function snakeToCamel(str: string): string {
   return str.replace(/_([a-z])/g, (_, letter) => letter.toUpperCase())
 }
 
-function isPlainObject(value: unknown): value is Record<string, unknown> {
-  return typeof value === "object" && value !== null && !Array.isArray(value)
-}
-
 export function objectToSnakeCase(
   obj: Record<string, unknown>,
   deep: boolean = true