Re-add love.filesystem.exists.

It's still useful sometimes because it's a bit cheaper than getInfo.

src/modules/filesystem/Filesystem.h

@@ -216,6 +216,11 @@ public:
 	 **/
 	virtual std::string getRealDirectory(const char *filename) const = 0;
 
+	/**
+	 * Gets whether anything exists at the specified path.
+	 **/
+	virtual bool exists(const char *filepath) const = 0;
+
 	/**
 	 * Gets information about the item at the specified filepath. Returns false
 	 * if nothing exists at the path.