Change parent type of `JsonProcessingException` to be `RuntimeException`

[cowtowncoder]

(NOTE: part of bigger set of changes, see https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-4)

Although choice of JsonProcessingException to extend IOException made sense originally (I think), later developments -- in particular, Java 8 Streams and functional approach in general -- have made use of checked exceptions cumbersome for many use cases.
But due to backwards compatibility concerns, change of base exception type has not been possible within 2.x releases.

Now that we are working on 3.0, however, we can make this change. Compared to alternative of providing alternate subtypes of ObjectMapper, ObjectReader and/or ObjectWriter (ones that do not declare checked exceptions for methods), this leads to simpler API.
For 2.x we may still consider simpler work-arounds (maybe convenience wrappers), but with 3.0 let's rebase.

Note, too, that this will help use of Jackson for some specific cases like serialization of this from toString() (and with #2176).

[cowtowncoder]

Note: actual change is being done via jackson-core, as:

FasterXML/jackson-core#661

and I have a branch with mostly complete changes. However, coordination with all modules is required so while this effort is finally active again, it may take a week or two.

[cowtowncoder]

A lot of work done, on exp/jstep-4-for-3.0 branches of:

• jackson-core, jackson-databind (core packages; annotations not affected)
• All dataformats (jackson-dataformats-binary, jackson-dataformats-text, jackson-dataformat-xml)
• jackson-jr
• Some datatypes (jackson-datatypes-collections, jackson-datatype-joda)
• Main modules (jackson-modules-base, jackson-modules-java8)

and just sent an email to jackson-dev.

Next steps:

• Continue work on modules (jackson-datatypes-misc, jackson-jaxrs-providers, especially)
• Coordinate with Kotlin and Scala module authors

and planning on merging the changes tomorrow, January 19th 2021, if all goes well.