T23c: AI chat, input

- PromptInput: attachments and context above the field, the owner's tools and
  send below; Enter sends, Shift-Enter breaks the line; / offers commands and
  @ offers context over forms::Suggestions; files dropped on it attach behind a
  drop veil
- AttachmentButton, AttachmentChip with thumbnails and uploads, ContextChips
- VoiceInputButton and VoiceWaveform with a red dot and the time
- ParameterPanel, SystemPromptEditor, TokenCounter, CostEstimator
- forms: a Newline action on Shift-Enter keeps one-line fields to one line;
  replace_trigger moves beside Suggestions; choose and dropped shared
- gallery: composer, pickers and tuning

src/documents/suggest.rs

@@ -4,6 +4,7 @@ use gpui::{App, ElementId, Entity, SharedString, Window};
 
 use crate::forms::{
     Choice, Pick, Suggestions, TextInput, active_trigger, emoji_found, handles_matching,
+    replace_trigger,
 };
 
 /// A trigger of the owner's: from the query and where it runs, the rows and what a pick does.
@@ -23,20 +24,6 @@ fn nothing() -> (Vec<Choice>, Pick) {
     )
 }
 
-/// Replaces the trigger and its query, `at..caret`, with `text`, as one undo step.
-pub(crate) fn replace_trigger(
-    field: &Entity<TextInput>,
-    at: usize,
-    caret: usize,
-    text: &str,
-    cx: &mut App,
-) {
-    field.update(cx, |input, cx| {
-        input.select(at..caret, cx);
-        input.insert(text, cx);
-    });
-}
-
 /// What `@` and `:` offer at the field's caret: people by handle, emoji by name; `extra` adds a trigger of the owner's, such as `/` in blocks.
 pub(crate) fn offers(
     id: ElementId,