Add 'latest' as alias for version 'x'

While it is possible to specify that you want the latest JDK version
by using semver X-Ranges (like 'x' for example), 'latest' is more
readable as explicitly states the intention.

README.md

@@ -36,6 +36,8 @@ Examples of version specifications that the java-version parameter will accept:
   
   e.g. ```8.0.x, >11.0.3, >=13.0.1, <8.0.212```
   
+  e.g. ```latest``` (same as ```x```)
+
 - An early access (EA) Java version
 
   e.g. ```14-ea, 15-ea```

src/installer.ts

@@ -271,6 +271,12 @@ function getDownloadInfo(
 }
 
 function normalizeVersion(version: string): string {
+  // 'latest' means the most recent of any JDK version
+  // semver won't match pre-release versions
+  if (version === 'latest') {
+    version = 'x';
+  }
+
   if (version.slice(0, 2) === '1.') {
     // Trim leading 1. for versions like 1.8
     version = version.slice(2);