Support adding file headers automatically via .editorconfig

When working with C# files, it's possible to specify [`file_header_template`](https://learn.microsoft.com/visualstudio/ide/reference/add-file-header?view=vs-2022) in an .editorconfig file and have a file header (banner comment with copyright or other licensing information) automatically added to new files and identified as an issue in existing files if missing. Despite what the linked doc says, this works in both VS and VS Code.

It would be useful to have a similar feature for bicep files either by leveraging `file_header_template` or some similar mechanism that allows a header to be specified in .editorconfig and then required on all bicep files in the workspace.

Start by reviewing the linked Visual Studio documentation for file_header_template and compare how .editorconfig handles file headers in Visual Studio and VS Code. Determine where Bicep workspace validation and new-file behavior are defined, then verify that a configured header is added to new Bicep files and reported when missing from existing files.

Written by the indexing model from the issue text.