Add dedicated actions for LSP completions insertion mode (#28121)

Adds actions so you can have customized keybindings for `insert` and
`replace` modes.

And add `shift-enter` as a default for `replace`, this will override the
default setting
`completions.lsp_insert_mode` which is set to `replace_suffix`, which
tries to "smartly"
decide whether to replace or insert based on the surrounding text.

For those who come from VSCode, if you want to mimic their behavior, you
only have to
set `completions.lsp_insert_mode` to `insert`.

If you want `tab` and `enter` to do different things, you need to remap
them, here is
an example:

```jsonc
[
  // ...
  {
    "context": "Editor && showing_completions",
    "bindings": {
      "enter": "editor::ConfirmCompletionInsert",
      "tab": "editor::ConfirmCompletionReplace"
    }
  },
]
```

Closes #24577

- [x] Make LSP completion insertion mode decision in guest's machine
(host is currently deciding it and not allowing guests to have their own
setting for it)
- [x] Add shift-enter as a hotkey for `replace` by default.
- [x] Test actions.
- [x] Respect the setting being specified per language, instead of using
the "defaults".
- [x] Move `insert_range` of `Completion` to the Lsp variant of
`.source`.
- [x] Fix broken default, forgotten after
https://github.com/zed-industries/zed/pull/27453#pullrequestreview-2736906628,
should be `replace_suffix` and not `insert`.

Release Notes:

- LSP completions: added actions `ConfirmCompletionInsert` and
`ConfirmCompletionReplace` that control how completions are inserted,
these override `completions.lsp_insert_mode`, by default, `shift-enter`
triggers `ConfirmCompletionReplace` which replaces the whole word.

crates/assistant_context_editor/src/slash_command.rs

@@ -120,7 +120,7 @@ impl SlashCommandCompletionProvider {
                                         ) as Arc<_>
                                     });
                             Some(project::Completion {
-                                old_range: name_range.clone(),
+                                replace_range: name_range.clone(),
                                 documentation: Some(CompletionDocumentation::SingleLine(
                                     command.description().into(),
                                 )),
@@ -219,7 +219,7 @@ impl SlashCommandCompletionProvider {
                             }
 
                             project::Completion {
-                                old_range: if new_argument.replace_previous_arguments {
+                                replace_range: if new_argument.replace_previous_arguments {
                                     argument_range.clone()
                                 } else {
                                     last_argument_range.clone()