bug#62400: 30.0.50; csharp-mode syntax highlight problem

[牟 桐]

Hi, I noticed that in `csharp-mode`, code like this will not be
highlighted correctly:

``` csharp-mode
using Stream stream
             |
             ^
           plain
```

The first character of `stream` is not highlighted.


In GNU Emacs 30.0.50 (build 2, x86_64-apple-darwin21.6.0, NS
appkit-2113.60 Version 12.6.3 (Build 21G419)) of 2023-03-17 built on
dalum.local
Repository revision: 0330cff65ae837e93ae4d059acf643734d16386d
Repository branch: master
Windowing system distributor 'Apple', version 10.3.2113
System Description:  macOS 12.6.3

Configured using:
'configure --with-native-compilation=aot'

Configured features:
ACL DBUS GIF GLIB GMP GNUTLS JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY KQUEUE NS PDUMPER PNG RSVG SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XIM ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: C#//l

Minor modes in effect:
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  recentf-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
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  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  size-indication-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
/Users/mou/.emacs.d/etc/abbrev hides 
/Applications/Emacs.app/Contents/Resources/lisp/abbrev

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Memory information:
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(strings 32 128156 65222)
(string-bytes 1 4379169)
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