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>

docs/src/configuring-zed.md

@@ -2051,6 +2051,68 @@ Examples:
 
 `boolean` values
 
+## Completions
+
+- Description: Controls how completions are processed for this language.
+- Setting: `completions`
+- Default:
+
+```json
+{
+  "completions": {
+    "words": "fallback",
+    "lsp": true,
+    "lsp_fetch_timeout_ms": 0,
+    "lsp_insert_mode": "replace_suffix"
+  }
+}
+```
+
+### Words
+
+- Description: Controls how words are completed. For large documents, not all words may be fetched for completion.
+- Setting: `words`
+- Default: `fallback`
+
+**Options**
+
+1. `enabled` - Always fetch document's words for completions along with LSP completions
+2. `fallback` - Only if LSP response errors or times out, use document's words to show completions
+3. `disabled` - Never fetch or complete document's words for completions (word-based completions can still be queried via a separate action)
+
+### LSP
+
+- Description: Whether to fetch LSP completions or not.
+- Setting: `lsp`
+- Default: `true`
+
+**Options**
+
+`boolean` values
+
+### LSP Fetch Timeout (ms)
+
+- Description: When fetching LSP completions, determines how long to wait for a response of a particular server. When set to 0, waits indefinitely.
+- Setting: `lsp_fetch_timeout_ms`
+- Default: `0`
+
+**Options**
+
+`integer` values representing milliseconds
+
+### LSP Insert Mode
+
+- Description: Controls what range to replace when accepting LSP completions.
+- Setting: `lsp_insert_mode`
+- Default: `replace_suffix`
+
+**Options**
+
+1. `insert` - Replaces text before the cursor, using the `insert` range described in the LSP specification
+2. `replace` - Replaces text before and after the cursor, using the `replace` range described in the LSP specification
+3. `replace_subsequence` - Behaves like `"replace"` if the text that would be replaced is a subsequence of the completion text, and like `"insert"` otherwise
+4. `replace_suffix` - Behaves like `"replace"` if the text after the cursor is a suffix of the completion, and like `"insert"` otherwise
+
 ## Show Completions On Input
 
 - Description: Whether or not to show completions as you type.