Clarify in the LazyOptional which methods carry the lazyness over to the returned value and which don't. (#6750)

For consistency, this meant making a few changes:
- Renamed the existing lazy mapping method to lazyMap, to indicate that it doesn't run the mapping immediately.
- Added a new implementation of map(), which returns Optional<T>, and resolves the value in the process.
- Changed filter() to return Optional, since there's no way to filter lazily.
- Added a new method resolve(), which helps convert the custom LazyOptional class into a standard Optional, for use
with library methods that expect Optional<T>.

* Update License headers.

src/main/java/net/minecraftforge/common/BiomeManager.java

@@ -16,6 +16,7 @@
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
+
 /* Biomes are completely redone in 1.16.2, reevaluate
 package net.minecraftforge.common;