Add Swift 5.3 to the Language Version filter

<https://apple.github.io/swift-evolution/#?version=5.3>

<#1114 (comment)>

Author: benrimmington

index.js

@@ -19,7 +19,7 @@ var proposals
  * To be updated when proposals are confirmed to have been implemented
  * in a new language version.
  */
-var languageVersions = ['2.2', '3', '3.0.1', '3.1', '4', '4.1', '4.2', '5', '5.1', '5.2']
+var languageVersions = ['2.2', '3', '3.0.1', '3.1', '4', '4.1', '4.2', '5', '5.1', '5.2', '5.3']
 
 /** Storage for the user's current selection of filters when filtering is toggled off. */
 var filterSelection = []
@@ -1039,4 +1039,4 @@ function cleanNumberFromState (state) {
 
 function addNumberToState (state, count) {
   return state + ' (' + count + ')'
-}
+}