runtime(csh): Update ftplugin, improve matchit behaviour

runtime(csh): Update ftplugin, improve matchit behaviour

- Allow for an unparenthesised expression argument to the 'if',
  'if-then', and 'while' commands.  This is undocumented, and probably
  unintended, behaviour but is frequently seen in the wild.
- Allow for a continued-line expression argument to the 'if-then'
  command.

related: #19172 (csh: Support negated if in matchit)
closes:  #19190

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>