Open excerpt on double click in multibuffer by default. (#9196)

Closes https://github.com/zed-industries/zed/issues/5275

Double click with `alt` modifier pressed will do the regular word
selection.

Adds a setting to disable this behavior and instead select a word, as in
the regular buffer.

```
// What to do when multibuffer is double clicked in some of its excerpts
// (parts of singleton buffers).
// May take 2 values:
//  1. Behave as a regular buffer and select the whole word.
//         "double_click_in_multibuffer": "select"
//  2. Open the excerpt clicked as a new buffer in the new tab (default).
//         "double_click_in_multibuffer": "open",
// For the case of "open", regular selection behavior can be achieved by holding `alt` when double clicking.
"double_click_in_multibuffer": "open",
```


Release Notes:

- Made multibuffer to open excerpts in new tabs on double click by
default (changing settings or keeping alt restores the word selection
behavior). ([5275](https://github.com/zed-industries/zed/issues/5275))