feat(highlight)!: error on invalid names and allow '.' and '@'

Previously if a highlight group with a name outside the regexp
[a-zA-Z0-9_] was defined, Nvim would emit an "invalid character"
warning message. This was annoying for Lua scripts, as it was very hard
to debug what line of code was triggering this message since it didn't
produce a stack trace.

This has now been promoted to an error with the code E5248.

Additionally the ASCII character period ('.') and at-sign ('@') have
been added to the allowed list of characters of a highlight group name
to support the application of defining hierarchical highlight groups,
e.g. 'TS.keyword'.

Co-authored-by: Christian Clason <christian.clason@uni-due.de>

1 files changed, 3 insertions, 0 deletions

diff --git a/runtime/doc/vim_diff.txt b/runtime/doc/vim_diff.txt
index 53effa1443..0011cd9821 100644
--- a/runtime/doc/vim_diff.txt
+++ b/runtime/doc/vim_diff.txt
@@ -390,6 +390,9 @@ Highlight groups:
   using |n| or |N|
   |hl-CursorLine| is low-priority unless foreground color is set
   |hl-VertSplit| superseded by |hl-WinSeparator|
+  Highlight groups names are allowed to contain the characters `.` and `@`.
+  It is an error to define a highlight group with a name that doesn't match
+  the regexp `[a-zA-Z0-9_.@]*` (see |group-name|).
 
 Macro/|recording| behavior
   Replay of a macro recorded during :lmap produces the same actions as when it