Improve F12 UX for using declaration in .bicepparam (#18372)

Updated `BicepDefinitionHandler` to handle `UsingDeclarationSyntax`,
ensuring that Bicep source files instead of ARM template files for
remote modules are opened (if Bicep source is available). This aligns
the user experience with the behavior of Go to Definition for module
paths in Bicep files.

Closes #17859.
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Author: shenglol

src/Bicep.LangServer/Handlers/BicepDefinitionHandler.cs

@@ -109,6 +109,17 @@ private LocationOrLocationLinks HandleUnboundSymbolLocation(DefinitionParams req
                     return HandleModuleReference(context, stringToken, sourceFile, moduleReference);
                 }
             }
+            {
+                // Handle using '<module_path>'
+                if (SyntaxMatcher.IsTailMatch<UsingDeclarationSyntax, StringSyntax, Token>(matchingNodes, (usingDeclaration, usingPath, token) => token.Type == TokenType.StringComplete) &&
+                    matchingNodes[^3] is UsingDeclarationSyntax usingDeclaration &&
+                    matchingNodes[^2] is StringSyntax stringSyntax &&
+                    context.Compilation.SourceFileGrouping.TryGetSourceFile(usingDeclaration).IsSuccess(out var sourceFile) &&
+                    moduleDispatcher.TryGetArtifactReference(context.Compilation.SourceFileGrouping.EntryPoint, usingDeclaration).IsSuccess(out var moduleReference))
+                {
+                    return HandleModuleReference(context, stringSyntax, sourceFile, moduleReference);
+                }
+            }
             { // Definition handler for a non symbol bound to implement import path goto.
                 // try to resolve import path syntax from given offset using tail matching.
                 if (SyntaxMatcher.IsTailMatch<CompileTimeImportDeclarationSyntax, CompileTimeImportFromClauseSyntax, StringSyntax, Token>(