Require global settings file to use pkl:settings (#477)

This is technically a breaking change, but follows the intended use-case
and documentation of settings.

* Require that the global settings file at `~/.pkl/settings.pkl`
amends stdlib module `pkl:settings`, or otherwise sets `output.value`
to an instance of `pkl.Settings`.
* Simplify the object mapping of `PklSettings.java`

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -700,9 +700,6 @@ Refusing to load module `{0}` because it does not match any entry in the module
 insufficientModuleTrustLevel=\
 Refusing to import module `{0}` because importing module `{1}` has an insufficient trust level.
 
-invalidSettingsFile=\
-Pkl settings file `{0}` must start with `amends "pkl:settings"`.
-
 invalidRegexSyntax=\
 Syntax error in regex `{0}`: {1}