It is not possible to integrate Notepad++ into the editor like in JNGP.
What I meant is what Drake53 suggested: You can code in Notepad++ and either use copy/paste or //! import
.
This is of course not the same as TESH and has several drawbacks, but it is a quick and easy solution if syntax highlighting is your priority.
TESH does not work anymore since Reforged. You can't install it with the new version.
The only way to use it, is to use an older version of Wc3 and the old JNGP or Sharpcraft. However, the maps will then only work with the old version of the game, so this is not a good solution.
The situation is a bit unfortunate.
In principle, I am pretty sure that TESH would still work with the World Editor, even in Reforged. However, TESH is not a standalone program, it is a dynamic-link library (dll). As such, it must be loaded by a host process in order to start.
Previously, JNGP loaded TESH on startup and by doing so, TESH is started. It does so, because JNGP loads a dll with the name TESH.dll automatically, by default (if such a dll exists). But since JNGP (and Sharcraft) don't work anymore since Reforged, there is no host process to load TESH and therefore it doesn't work anymore.
The problem could be fixed I guess by simpliy creating some executable like the one from JNGP that loads TESH.dll into the WE (but I don't know how to do that).