Reload language server when hdl-prj.json file changes

[eine]

Related to #4 and #5.

Reproduction steps:

• An hdl-prj.json is provided which contains paths to non-existing files.
• A message is shown for each not found file, which is correct.
• The hdl-prj.json is opened in VSC and it is fixed.
• Nothing happens.

See #4 (comment) for examples of valid and invalid hdl-prj.json files for VUnit's array_axis_vcs example.

I would expect hdl-prj.json to be reloaded (parsed again) after it is saved.

Unfortunately, VSC does not provide a 'reload language server' feature. It needs to be implemented in the extension. See microsoft/vscode#76405