Autocomplete of package name starting with "@" produces double @

[dimfeld]

Describe the bug
When performing import statement autocomplete on a namespaced package name (i.e. one starting with @) the completed package name ends up with a double @. This only happens in Svelte files, not in JS/TS files.

To Reproduce
Steps to reproduce the behavior:

1. Create a project with a namespaced package dependency. SvelteKit works well here since it includes @sveltejs/kit.
2. Open a Svelte file and type import * as p from '@.
3. Select @sveltejs/kit or any other namespaced package and accept the suggestion.
4. Notice that the final line is import * as kit from '@@sveltejs/kit' with the @ doubled up.

This feels like an issue of the @ character just not being treated as part of the "existing text" in the autocomplete.

Expected behavior
Just a single @ character in the package name.

System (please complete the following information):

MacOS 12.01 on M1, VS Code 1.61.2, Svelte for VS Code 105.5.0

But the versions don't matter much. I first noticed this in Neovim on Ubuntu Linux with coc.nvim/coc-svelte a few months ago and assumed it was just a Vim problem until I tried to reproduce it in VSCode today and saw it happens there too.

[jasonlyu123]

The reason for this and #1211 is that some editors would check the word at the position of the completion to filter out duplicated characters. And it uses a pattern for identifier or syntax keyword. But completion in a string literal is likely to have something that doesn't match the pattern.