Fix bug when loglevel is used with some ESM translation (e.g. ts-node)

Although loglevel itself is not published using ESM, it seems that some
runtime environments will treat its import as such. Because of this,
when it's imported with syntax like "import * as log ...", all fields of
loglevel are treated as ESM exports, meaning they're frozen and
read-only.

This is problematic, because loglevel rewrites those fields to change
levels, which now fails.

This happened in v1.9.0 because we switched from replacing log methods
on 'self' to 'this'. 'self' always referred to the logger object within
our environment, but 'this' in these runtime environments seems to point
to the ESM-imported wrapper which is unmodifiable. Using the internal
self reference to our internal state seems to work correctly instead,
with no downsides, so we're reverting to that.

lib/loglevel.js

@@ -264,7 +264,7 @@
           }
 
           // NOTE: in v2, this should call rebuild(), which updates children.
-          return replaceLoggingMethods.call(this);
+          return replaceLoggingMethods.call(self);
       };
 
       self.setDefaultLevel = function (level) {
@@ -277,7 +277,7 @@
       self.resetLevel = function () {
           userLevel = null;
           clearPersistedLevel();
-          replaceLoggingMethods.call(this);
+          replaceLoggingMethods.call(self);
       };
 
       self.enableAll = function(persist) {
@@ -292,7 +292,7 @@
           if (defaultLogger !== self) {
               inheritedLevel = normalizeLevel(defaultLogger.getLevel());
           }
-          replaceLoggingMethods.call(this);
+          replaceLoggingMethods.call(self);
 
           if (defaultLogger === self) {
               for (var childName in _loggersByName) {
@@ -309,7 +309,7 @@
       if (initialLevel != null) {
           userLevel = normalizeLevel(initialLevel);
       }
-      replaceLoggingMethods.call(this);
+      replaceLoggingMethods.call(self);
     }
 
     /*