Tighten accessor naming rules to not allow leading lower-case or non-letter character for getters/setters

[cowtowncoder]

(follow-up to #2729)

Jackson 2.x allows accessors names like:

• get_value(), island(), setter() (to indicate properties _value, land and ter, respectively)

although does allow ways to change these defaults via AccessorNamingStrategy (since #2800 added for 2.12).
This is due to backwards-compatibility.

But with 3.0 we can change the restrictions and it seems reasonable to default to not including examples like listed above.

[cowtowncoder]

Funny: support for this has been available since 2.12. Quite easy to fix after all.