fix: The boundary letter has not been respected during node list pattern matching.

Retrieving the boundary letter from a spot object has been wrong in
some places.  The boundary letter is not stored in a field .boundary,
but .boundary_letter.

file                                                       introduced
lua-modules/pdnm_nl_manipulation.lua                       ff96784, 69ca5d3
examples/luatex/pdnm_hyphenate-with-explicit-hyphen.lua    875dd25
example: Refactor.

Move example pattern files to central directory example/patterns/.
Write .spots file with debug hyphenation information to working directory.

Prevent from writing .spots file into the same directory as that where
patterns have been read from, as that might be a system directory.
example: non-standard-hyphenation-german: Refactor.

Rename Lua module.
example: non-standard-hyphenation-german: Fix spelling errors.
example: hyphenate-with-explicit-hyphen: Use dedicated compound word hyphenation patterns.
example: patternize: Refactor.

Split script into frontend and backend.
class: cls_pdnm_spot: Make spot characters properties of spot instance.
example: patternize: Provide option --chars for setting more special characters.
example: patternize: Refactor.

Revise option handling.
example: patternize: New option for determining whether matching patterns are to be shown.