Add completions.lsp_insert_mode setting to control what ranges are replaced when a completion is inserted (#27453)

This PR adds `completions.lsp_insert_mode` and effectively changes the
default from `"replace"` to `"replace_suffix"`, which automatically
detects whether to use the LSP `replace` range instead of `insert`
range.

`"replace_suffix"` was chosen as a default because it's more
conservative than `"replace_subsequence"`, considering that deleting
text is usually faster and less disruptive than having to rewrite a long
replaced word.

Fixes #27197
Fixes #23395 (again)
Fixes #4816 (again)

Release Notes:

- Added new setting `completions.lsp_insert_mode` that changes what will
be replaced when an LSP completion is accepted. The default is
`"replace_suffix"`, but it accepts 4 values: `"insert"` for replacing
only the text before the cursor, `"replace"` for replacing the whole
text, `"replace_suffix"` that acts like `"replace"` when the text after
the cursor is a suffix of the completion, and `"replace_subsequence"`
that acts like `"replace"` when the text around your cursor is a
subsequence of the completion (similiar to a fuzzy match). Check [the
documentation](https://zed.dev/docs/configuring-zed#LSP-Insert-Mode) for
more information.

---------

Co-authored-by: João Marcos <marcospb19@hotmail.com>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/copilot/src/copilot_completion_provider.rs

@@ -273,7 +273,7 @@ mod tests {
     use language::{
         Point,
         language_settings::{
-            AllLanguageSettings, AllLanguageSettingsContent, CompletionSettings,
+            AllLanguageSettings, AllLanguageSettingsContent, CompletionSettings, LspInsertMode,
             WordsCompletionMode,
         },
     };
@@ -294,6 +294,7 @@ mod tests {
                 words: WordsCompletionMode::Disabled,
                 lsp: true,
                 lsp_fetch_timeout_ms: 0,
+                lsp_insert_mode: LspInsertMode::Insert,
             });
         });
 
@@ -525,6 +526,7 @@ mod tests {
                 words: WordsCompletionMode::Disabled,
                 lsp: true,
                 lsp_fetch_timeout_ms: 0,
+                lsp_insert_mode: LspInsertMode::Insert,
             });
         });