[Theme request] soft-era

### Describe your request

[soft-era](https://github.com/soft-aesthetic/soft-era) is a cute, light theme. nobody implement it for emacs.
It website is done and looks like nobody maintains the project (soft-era).
lucky, vscode and vim repo's LICENSE is MIT.
So, request the theme.

### Resources

- main website is [~ soft aesthetic ~](https://github.com/soft-aesthetic)
- vscode theme repo is [soft-era-vs-code](https://github.com/soft-aesthetic/soft-era-vs-code)
- vim theme repo is [soft-era-vim](https://github.com/soft-aesthetic/soft-era-vim)

Compare the soft-era-vs-code and soft-era-vim repositories linked in the issue, then inspect existing themes in this repository to identify the expected Emacs theme entry points. The work is done when the soft-era theme is added in the repository's established format and matches the referenced theme's appearance.

Written by the indexing model from the issue text.