This PR changes the approach we're using to render the channel buttons
to use a more straightforward (and less hacky) approach.

### Motivation

Even with the variety of hacks that were employed to make the current
approach work, there are still a number of issues with the current
solution:

- Hovering in the empty space to the left of a channel doesn't correctly
apply the hover background to the container for the channel buttons
- Hovering to the very right of the collab panel (right on top of the
drag handle) causes the channel button container to apply its hover
background without applying it to the rest of the row
- The buttons would still get pushed off to the right by the indent
space in the channel tree, resulting in jagged indicators at small sizes

Additionally, the rectangular background placed behind the channel
buttons still didn't look great when it overlapped with the channel
names.

### Explanation

For these reasons, I decided to explore a simpler approach that
addresses these issues, albeit with some tradeoffs that I think are
acceptable.

We now render the absolutely-positioned button container as a sibling
element of the `ListItem`. This is to avoid issues with the container
getting pushed around based on the contents of the `ListItem` rather
than staying absolutely positioned at the end of the row.

We also have gotten rid of the background for the button container, and
now rely on the background of the individual `IconButton`s to occlude
the channel name behind them when the two are overlapping.

Here are some examples of the new UI in various configurations:

#### When the channel entry is hovered

<img width="270" alt="Screenshot 2024-01-09 at 6 15 24 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0207a129-30eb-4067-8490-3b6c9cbf75ea">

#### Overlapping with the channel name

<img width="229" alt="Screenshot 2024-01-09 at 6 15 43 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0a67507e-45c8-4325-a71e-d416edc3a115">

#### Narrow collab panel

<img width="116" alt="Screenshot 2024-01-09 at 6 16 07 PM"
src="https://github.com/zed-industries/zed/assets/1486634/887ebaa3-e4d6-4497-9141-afcc8b7cd356">

### Tradeoffs

The new approach comes with the following tradeoffs that I am currently
aware of:

The occlusion can look a little weird when the icons are in the middle
of a channel name (as opposed to fully occluding the end of the channel
name):
<img width="190" alt="Screenshot 2024-01-09 at 6 24 32 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0062b806-1b8f-47eb-af8d-f061d9829366">

Hovering one of the icons causes the icon to be hovered instead of the
row:

<img width="232" alt="Screenshot 2024-01-09 at 6 31 38 PM"
src="https://github.com/zed-industries/zed/assets/1486634/cbcc1a45-64d2-4890-8ad6-c5a5ee961b86">

Release Notes:

- Improved the way channel buttons are displayed.