runtime(vim): Cleanup syntax tests

Improve formatting and naming consistency of the syntax tests.

closes: #17850

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

runtime/syntax/testdir/input/vim9_comments.vim

@@ -0,0 +1,92 @@
+vim9script
+# Vim9 comments
+
+
+# Vim9-script comment
+
+ # string only recognised with leading char
+ "useless string"
+
+function! Foo()
+  " Legacy-script comment
+  # 42 " comment
+endfunction
+
+def! Bar()
+  # Vim9-script comment
+  "useless string" # comment
+enddef
+
+command -count FooCommand {
+  # Vim9-script comment
+  "useless string" # comment
+}
+
+autocmd BufNewFile * {
+  # Vim9-script comment
+  "useless string" # comment
+}
+
+
+# Multiline comments
+
+# comment
+  \ continuing comment
+  \ continuing comment
+
+var foo = 42 # commment
+  \ continuing comment
+  \ continuing comment
+
+# :Foo
+      \ arg1
+      #\ comment
+      \ arg2
+
+echo "TOP"
+
+
+# Line-continuation comments
+
+:Foo
+      #\ line continuation comment
+      \ arg1
+      #\ line continuation comment
+      \ arg2
+
+
+# Expression comments
+
+var foo = { # comment
+  # comment
+  bar: 42, # comment
+  # comment
+  # comment
+  baz: 42 # comment
+  # comment
+} # comment
+
+
+# Issue: #13047
+
+if !exists(":DiffOrig")
+  command DiffOrig vert new | set bt=nofile | r ++edit %% | 0d_ | diffthis
+		  \ | wincmd p | diffthis
+endif
+
+
+# Issue: #11307 and #11560
+
+# This is what we call " blah
+
+
+# PR: #14975
+# https://github.com/vim/vim/pull/14975#issuecomment-2832643115
+
+var d = {
+    a: 0,
+# a ' quote {{{
+#}}}
+b: 0,
+}
+