Add better support for multiple top-level values

[Marcono1234]

Purpose

Add better support for reading and writing multiple top-level values

• Resolves JsonReader with strictness set to LENIENT, #2769
• Relates to Making Gson.fromJson(Reader, Class) return after one JSON value is read #1733 (see comments there)
• Obsoletes bug fix issue #2769 #2779
  (the user can now customize the separator between top-level values, so that is preferred over making JsonReader tell more ambiguous concatenated top-level value apart)

Description

Currently reading and writing multiple top-level JSON values is coupled with lenient mode. This might be undesired for users who normally want to use strict mode.
Having multiple top-level values is a common use case when streaming JSON values, see for example JSON Lines. And another problem is that JsonWriter currently concatenates multiple JSON values without any separating whitespace, and without any way to configure it.

Other JSON libraries seem to have dedicated classes or configuration for multiple top-level value support, e.g. Jackson's MappingIterator and SequenceWriter and kotlinx-serialization's DecodeSequenceMode.WHITESPACE_SEPARATED.
However, because Gson's JsonReader and JsonWriter already had (limited) functionality for multiple top-level values, I added new API to those classes instead of adding completely new classes.

Exposing this also through Gson / GsonBuilder might not be needed since the methods of the Gson class are mainly for reading and writing a single value.

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[Marcono1234]

I am not completely happy about these method names setMultiTopLevelValuesAllowed and isMultiTopLevelValuesAllowed yet. Any feedback and suggestions are welcome.

[Marcono1234]

Moved this pos--; outside the if-else since the if { ... } performs a return.

[Marcono1234]

Looks like this "fall-through" was previously redundant: The next case is EMPTY_DOCUMENT which replaces the top of the stack with NONEMPTY_DOCUMENT, but here the top is already NONEMPTY_DOCUMENT.

[Marcono1234]

An alternative would be to always enable support for multiple top-level values, regardless of strictness, and then only have JsonWriter#setTopLevelSeparator(String) to allow customizing the separator when writing.

Though since there are multiple formats / standards for encoding multiple top-level values, maybe this is not desired? And could cause confusion when users accidentally write or read multiple top-level values? (though a bit unlikely?)