Refactor and improve completion from nowhere or manual trigger

- Regroup completion cases under `Context` in a OOP-like manner to
  simplify parameter passing.

- Handle more cases for complete from nowhere (between expressions or
  bindings). This helps 24x7 completion and manually triggered
  completion (Ctrl-Space) in various places to list all options without
  any identifier as filter.

crates/ide/src/ide/mod.rs

@@ -164,7 +164,7 @@ impl Analysis {
         &self,
         pos: FilePos,
         trigger_char: Option<char>,
-    ) -> Cancellable<Option<Vec<CompletionItem>>> {
+    ) -> Cancellable<Vec<CompletionItem>> {
         self.with_db(|db| completion::completions(db, pos, trigger_char))
     }
 

crates/nil/src/convert.rs

@@ -147,7 +147,7 @@ pub(crate) fn to_completion_item(line_map: &LineMap, item: CompletionItem) -> ls
         // We don't support indentation yet.
         insert_text_mode: Some(lsp::InsertTextMode::ADJUST_INDENTATION),
         text_edit: Some(lsp::CompletionTextEdit::Edit(lsp::TextEdit {
-            range: to_range(line_map, item.source_range),
+            range: to_range(line_map, item.replace_range),
             new_text: item.replace.into(),
         })),
         detail: item.description,

crates/nil/src/handler.rs

@@ -76,9 +76,7 @@ pub(crate) fn completion(
     let trigger_char = params
         .context
         .and_then(|ctx| ctx.trigger_character?.chars().next());
-    let Some(items) = snap.analysis.completions(fpos, trigger_char)? else {
-        return Ok(None);
-    };
+    let items = snap.analysis.completions(fpos, trigger_char)?;
     let items = items
         .into_iter()
         .map(|item| convert::to_completion_item(&line_map, item))