Add icons for Nim project files

[fyrak1s]

It would be awesome if icons could be added for Nim lang projects :))

The file endings in nim projects are .nim, .nims (nimscript), and .nimble (the package manager).

The material theme for instance uses this icon for all of those file endings: https://github.com/PKief/vscode-material-icon-theme/blob/main/icons/nim.svg

Thank you in advance =)