Annoying completion result for options value

**Describe the bug**
A clear and concise description of what the bug is.

vim-language-server returns annoying completion results when we trigger completion in value of options.
Even the option does not accept vim script expression (e.g. `'encoding'`, `'formatoptions'`), LS suggests vim script variables.

**To Reproduce**
Steps to reproduce the behavior:
1. Start completion with leading text like `set encoding=`

**Expected behavior**

vim-language-server should return the result following context.

**Screenshots**

**Desktop (please complete the following information):**
- OS: macOS Sonoma 14.5