misc/notepadplus: use new User Defined Language system (UDL2)

Add missing single quotation and backslash marks.

Change dot and underscore character keyword type.
"_" is a predeclared identifier, not a operator.
"." is a selector, x.f should be one identifier highlight.
So the fix is to change it.

Fixes issue 5775.
Fixes issue 5788.
Fixes issue 5798.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/10480044

Committer: Rob Pike <r@golang.org>