Fix IntelliJ Java warnings, make classes final if possible

[translatenix]

Fix all IntelliJ warnings in pkl-core production code except for bogus spelling warnings and warnings about unused public methods. I didn't delete any unused public method because it's hard to tell for sure which packages are public APIs.

Changes made:

• use text block instead of string concatenation
• extract method to avoid code duplication
• use switch expression
• fix Javadoc syntax and spelling
• fix spelling in comment
• increase class visibility to match visibility of use site
• delete overriding method with same implementation
• use String.isEmpty() and StringBuilder.isEmpty()
• add @Serial annotation
• make field final
• remove unused field
• remove unused private method
• remove exceptions that aren't thrown from throws clause
• insert non-null assertion
• annotate overriding method with @Nonnull
• suppress warning
• delete unused class (WriteAuxiliarySlotNode)

[holzensp]

Overall, LGTM... Why does my IntelliJ not warn about lacking / suggest adding final modifiers to classes?

[translatenix]

Why does my IntelliJ not warn about lacking / suggest adding final modifiers to classes?

Unlike the other commits, this one doesn’t address IntelliJ warnings (see commit messages). It fixes gaps in making classes final, a policy that is generally adhered to in the codebase.

[translatenix]

You need to fix some formatting problems to get a green build, though.

Fixed by rebasing.

[translatenix]

Squashed the "fix IntelliJ warnings" commits.