patch 9.1.2027: filetype: bicep filetype used for 2 bicep file types

Problem:  filetype: bicep filetype used for 2 bicep file types
Solution: Detect *.bicepparam files as bicep-param filetype, include
          new bicep-params and bicep filetype plugin
          (Scott McKendry)

The bicep language server handles parameter files differently than
regular bicep files. Treating them the same at the editor level leads to
false positive diagnostics in the editor.

References:

Bicep Language Constants:
- 51392d32ca/src/Bicep.Core/LanguageConstants.cs (L23)
VS Code Extension definition:
- 51392d32ca/src/vscode-bicep/package.json (L47)

closes: #19026

Signed-off-by: Scott McKendry <me@scottmckendry.tech>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/ftplugin/bicep-params.vim

@@ -0,0 +1,3 @@
+" Placeholder for maximum compatibility. While bicepparam files are treated as distinct filetypes,
+" they are should share the same ftplugin settings as bicep files.
+runtime! ftplugin/bicep.vim

runtime/ftplugin/bicep.vim

@@ -0,0 +1,14 @@
+" Vim filetype plugin
+" Language:	Bicep
+" Maintainer:	Scott McKendry <me@scottmckendry.tech>
+" Last Change:	2025 Dec 27
+
+if exists('b:did_ftplugin')
+  finish
+endif
+let b:did_ftplugin = 1
+
+setlocal comments=s1:/*,mb:*,ex:*/,://
+setlocal commentstring=//\ %s
+
+let b:undo_ftplugin = "setlocal comments< commentstring<"