Make picocli trace levels case-insensitive

[remkop]

Currently, only values WARN, INFO and DEBUG, and empty String or case-insensitive "true" are valid values for the system property picocli.trace.

We can make this more user-friendly by allowing values WARN, INFO and DEBUG to be case-insensitive.

Index: src/main/java/picocli/CommandLine.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- src/main/java/picocli/CommandLine.java	(date 1594795763490)
+++ src/main/java/picocli/CommandLine.java	(date 1594795763490)
@@ -16896,7 +16896,7 @@
             if (tracer.level.isEnabled(this)) { tracer.stream.printf(prefix(msg), params); }
         }
         private String prefix(String msg) { return "[picocli " + this + "] " + msg; }
-        static TraceLevel lookup(String key) { return key == null ? WARN : empty(key) || "true".equalsIgnoreCase(key) ? INFO : valueOf(key); }
+        static TraceLevel lookup(String key) { return key == null ? WARN : empty(key) || "true".equalsIgnoreCase(key) ? INFO : valueOf(key.toUpperCase(ENGLISH)); }
     }
     static class Tracer {
         TraceLevel level = TraceLevel.lookup(System.getProperty("picocli.trace"));