Rename _mousePressed, _keyPressed, _frameRate to match actual Processing.h variable names

Author: pepc84

assets/reference/data/frameRate_var.json

@@ -1,10 +1,10 @@
 {
   "slug": "frameRate_var",
-  "name": "frameRate",
+  "name": "_frameRate",
   "cat": "Environment",
   "subcat": "",
   "desc": "System variable storing the approximate current frame rate.",
-  "syntax": "frameRate",
+  "syntax": "_frameRate",
   "params": [],
   "returns": "float",
   "related": [

assets/reference/data/keyPressed_var.json

@@ -1,10 +1,10 @@
 {
   "slug": "keyPressed_var",
-  "name": "keyPressed",
+  "name": "_keyPressed",
   "cat": "Input",
   "subcat": "Keyboard",
   "desc": "The boolean system variable keyPressed is true if any key is pressed.",
-  "syntax": "keyPressed",
+  "syntax": "_keyPressed",
   "params": [],
   "returns": "bool",
   "related": [

assets/reference/data/mousePressed_var.json

@@ -1,10 +1,10 @@
 {
   "slug": "mousePressed_var",
-  "name": "mousePressed",
+  "name": "_mousePressed",
   "cat": "Input",
   "subcat": "Mouse",
   "desc": "Variable storing whether any mouse button is currently pressed.",
-  "syntax": "mousePressed",
+  "syntax": "_mousePressed",
   "params": [],
   "returns": "bool",
   "related": [