fix(help): fix CTRL character issue for :help {subject} #39537

Problem:
The argument to `:help` is normalized to fit the general tag format.
I.e. i^U-default, iCTRL-U-default and i_CTRL_U-default should all point
to the i_CTRL_U-default tag. Our normalization adds an underscore around
the CTRL keycode, e.g. iCTRL-GCTRL-J becomes i_CTRL-G_CTRL-J. That's not
necessary if the following part starts with a dash, like the case of
iCTRL-U-default.

Solution:
Do not insert an underscore if the following character is a dash/minus
(-).

runtime/lua/vim/_core/help.lua

@@ -108,7 +108,7 @@ function M.escape_subject(word)
     -- E.g. 'iCTRL-GCTRL-J' --> 'i_CTRL-G_CTRL-J'
     -- Only exception: 'CTRL-{character}'
     word = word:gsub('([^_])CTRL%-', '%1_CTRL-')
-    word = word:gsub('(CTRL%-[^{])([^_\\])', '%1_%2')
+    word = word:gsub('(CTRL%-[^{])([^-_\\])', '%1_%2')
 
     -- Skip function arguments
     -- E.g. 'abs({expr})' --> 'abs'