Release Notes:

- Improved "Extensions" UI details to tidy layout, add border, add
placeholder to search input.


## Before


![SCR-20240229-fbk](https://github.com/zed-industries/zed/assets/5518/a62608e2-8f79-48fe-9e1c-87d6eb59ddc9)

## After


![SCR-20240229-fhz](https://github.com/zed-industries/zed/assets/5518/d5c97711-ff93-4944-8e23-39b60dda88fc)

![SCR-20240229-fg9](https://github.com/zed-industries/zed/assets/5518/a24ad839-3b69-4ca7-813f-00375a81a008)

![SCR-20240229-fbb](https://github.com/zed-industries/zed/assets/5518/ff45d6c3-93a3-431c-81f5-edc9e7aa68d6)

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Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>