Make word completions less intrusive (#36745)

Introduce `min_words_query_len` threshold for automatic word completion
display, and set it to 3 by default.

Re-enable word completions in Markdown and Plaintext.

Release Notes:

- Introduced `min_words_query_len` threshold for automatic word
completion display, and set it to 3 by default to make them less
intrusive

crates/copilot/src/copilot_completion_provider.rs

@@ -301,6 +301,7 @@ mod tests {
         init_test(cx, |settings| {
             settings.defaults.completions = Some(CompletionSettings {
                 words: WordsCompletionMode::Disabled,
+                words_min_length: 0,
                 lsp: true,
                 lsp_fetch_timeout_ms: 0,
                 lsp_insert_mode: LspInsertMode::Insert,
@@ -533,6 +534,7 @@ mod tests {
         init_test(cx, |settings| {
             settings.defaults.completions = Some(CompletionSettings {
                 words: WordsCompletionMode::Disabled,
+                words_min_length: 0,
                 lsp: true,
                 lsp_fetch_timeout_ms: 0,
                 lsp_insert_mode: LspInsertMode::Insert,