Adds a new header `pluginexport.h` to put the macros in, could be
moved into an existing header (support.h?) by I didn't want to drag
a bunch of existing stuff into the source files for this one macro.

TagManager has relative include, this could be fixed by changing the
include directories for it if it's a problem. Mark the Scintilla
functions exported by re-declaring them in sciwrappers.c with the
attribute to avoid changing upstream Scintilla code.