bug: text with diacritic marks does not highlight properly

[ollien]

Check the following:

• I have checked the behavior after setting "vscode-neovim.neovimClean" in VSCode settings.
• I have read the vscode-neovim docs.
• I have searched existing issues.

Neovim version (nvim -v)

v0.9.5

Operating system/version

Fedora 39

Describe the bug

Text with diacritic marks do not highlight their full character width. This mostly affects the subset of unicode where we combine base letters with diacritic marks (i.e. what Zalgo text uses; https://en.wikipedia.org/wiki/Combining_Diacritical_Marks), rather than the standalone characters like à.

I'll admit that this is likely not that prevalent in code, but I found it by accident and figured it was better in a bug tracker than in my head. I suspect this is somewhat related to #1367

Steps To Reproduce

1. Create a document with the following content

ȯo ȯo
őo őo

2. Place your cursor on the first character of any line, and press *
3. Notice that the highlight is a narrow width, rather than the actual character width

Expected Behavior

The entire character should be highlighted. Plain neovim handles this case correctly