To implement peek()

To implement peek() as in https://github.com/vshymanskyy/TinyGSM/issues/396

src/TinyGsmFifo.h

@@ -122,6 +122,11 @@ public:
         return n - c;
     }
 
+	uint8_t peek()
+	{
+		return _b[_r];
+	}
+
 private:
     int _inc(int i, int n = 1)
     {

src/TinyGsmTCP.tpp

@@ -241,10 +241,9 @@ class TinyGsmTCP {
       return -1;
     }
 
-    // TODO(SRGDamia1): Implement peek
-    int peek() override {
-      return -1;
-    }
+	int peek() override {
+		return (uint8_t)rx.peek();
+	}
 
     void flush() override {
       at->stream.flush();