Transform plugin removes throwable information

In a simple use case, e.g.:
```
logger.error("Hello, {}!", "Problem", new Exception("Oh oh"));
```

Without the transform plugin, the default parameterized message formatter captures the last argument as a throwable -
if it's the last argument and the previous argument count match the placeholder count.
When processed by the `ExtendedThrowablePatternConverter`, it is fetched -
from `LogEvent.getThrown()`, which was previously populated when the LogEvent was created, from `Message.getThrowable()`.
This provides the throwable pattern converter the information to format the throwable as a message/stacktrace.

Here is the code generated by the transform plugin:
```
Logger var10000 = logger;
Object log4j2$$p0 = new Exception("Oh oh");
Object log4j2$$p1 = "Problem";
Object log4j2$$p2 = "Hello, {}!";
var10000.atError().withLocation(Cache.locations[0]).log(log4j2$$p2, log4j2$$p1, log4j2$$p0);
```
It create a `LogBuilder`, but does not use its `.withThrowable(Throwable)` method -
leading to a `LogEvent` that does not have the throwable set.

My thought is that to resolve that, the transform plugin needs - at build time - to analyze the logger call *in the same way the logger does* to extract the throwable arg and set it using `.withThrowable(Throwable)`.
An alternative (might be simpler) - is to change the `DefaultLogBuilder` (in `log4j-api`), so that `DefaultLogBuilder.logMessage(Message)` changes from
```
logger.logMessage(level, marker, fqcn, location, message, throwable);
```
to something like
```
logger.logMessage(level, marker, fqcn, location, message, throwable != null ? throwable : message.getThrowable());
```