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/language/src/language_settings.rs

@@ -61,7 +61,7 @@ pub fn all_language_settings<'a>(
 pub struct AllLanguageSettings {
     /// The edit prediction settings.
     pub edit_predictions: EditPredictionSettings,
-    defaults: LanguageSettings,
+    pub defaults: LanguageSettings,
     languages: HashMap<LanguageName, LanguageSettings>,
     pub(crate) file_types: HashMap<Arc<str>, GlobSet>,
 }
@@ -329,6 +329,11 @@ pub struct CompletionSettings {
     /// Default: 0
     #[serde(default = "default_lsp_fetch_timeout_ms")]
     pub lsp_fetch_timeout_ms: u64,
+    /// Controls how LSP completions are inserted.
+    ///
+    /// Default: "replace_suffix"
+    #[serde(default = "default_lsp_insert_mode")]
+    pub lsp_insert_mode: LspInsertMode,
 }
 
 /// Controls how document's words are completed.
@@ -345,10 +350,29 @@ pub enum WordsCompletionMode {
     Disabled,
 }
 
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, PartialEq, Eq, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum LspInsertMode {
+    /// Replaces text before the cursor, using the `insert` range described in the LSP specification.
+    Insert,
+    /// Replaces text before and after the cursor, using the `replace` range described in the LSP specification.
+    Replace,
+    /// Behaves like `"replace"` if the text that would be replaced is a subsequence of the completion text,
+    /// and like `"insert"` otherwise.
+    ReplaceSubsequence,
+    /// Behaves like `"replace"` if the text after the cursor is a suffix of the completion, and like
+    /// `"insert"` otherwise.
+    ReplaceSuffix,
+}
+
 fn default_words_completion_mode() -> WordsCompletionMode {
     WordsCompletionMode::Fallback
 }
 
+fn default_lsp_insert_mode() -> LspInsertMode {
+    LspInsertMode::Insert
+}
+
 fn default_lsp_fetch_timeout_ms() -> u64 {
     0
 }