file_icons: Resolve icon properly for exact file name match (#24396)

Follow-up to #24391 

The current approach has two issues:
- For the described case of `eslint.config.js`, for which a mapping
exists in `suffixes`, this would get mapped from `eslint.config.js` to
`eslint`. However, for `eslint`, there is no mapping within `suffixes`,
thus currently `get_icon_from_suffix` would return `None` and a wrong
item would be returned at a later step.
- Paths passed to this method are relative to the worktree root, thus
e.g. `eslint.config.js` files in subdirectories would still be assigned
the wrong icon.

---

Behaviour on `main`:
<img width="281" alt="main"
src="https://github.com/user-attachments/assets/19b5e5f8-e413-4ac9-a0a1-2c72f810aa86"
/>

Behaviour with this change:
<img width="299" alt="pr"
src="https://github.com/user-attachments/assets/eec70cbd-df39-49b4-8b07-d22afa949781"
/>

CC @probably-neb 

Release Notes:

- N/A