When using VSCodium (or VSCode) with Sumneko Lua plugin, definitions of 3rd part API have to be integrated.
Here you will find definitions of functions and classes for Wireshark Lua API.
This code is not integrated in Sumneko repository yet but it is ready to be copied in ~\.vscode-oss\extensions\sumneko.lua-2.5.3\server\meta\3rd folder for VSCodium.
It should be ~\.vscode\extensions\sumneko.lua-2.5.3\server\meta\3rd folder for VSCode.
On Windows, look in %USERPROFILE%.
Otherwise, put these files in your workspace or put the path in Lua.workspace.library variable in Sumneko extension settings.