compose: Implement inlineLink, for Markdown "inline link" syntax

And use it in the one place where we've been creating inline links.

This is a small amount of code, but drawing it out into a helper
gives a convenient place to write down its shortcomings.

We'll also use this for zulip#116 quote-and-reply, coming up.

lib/model/compose.dart

@@ -180,3 +180,25 @@ Uri narrowLink(PerAccountStore store, Narrow narrow, {int? nearMessageId}) {
 String mention(User user, {bool silent = false}) {
   return '@${silent ? '_' : ''}**${user.fullName}|${user.userId}**';
 }
+
+/// https://spec.commonmark.org/0.30/#inline-link
+///
+/// The "link text" is made by enclosing [visibleText] in square brackets.
+/// If [visibleText] has unexpected features, such as square brackets, the
+/// result may be surprising.
+///
+/// The part between "(" and ")" is just a "link destination" (no "link title").
+/// That destination is simply the stringified [destination], if provided.
+/// If that has parentheses in it, the result may be surprising.
+// TODO: Try harder to guarantee output that creates an inline link,
+//   and in particular, the intended one. We could help with this by escaping
+//   square brackets, perhaps with HTML character references:
+//     https://github.com/zulip/zulip-flutter/pull/201#discussion_r1237951626
+//   It's also tricky because nearby content can thwart the inline-link syntax.
+//   From the spec:
+//   > Backtick code spans, autolinks, and raw HTML tags bind more tightly
+//   > than the brackets in link text. Thus, for example, [foo`]` could not be
+//   > a link text, since the second ] is part of a code span.
+String inlineLink(String visibleText, Uri? destination) {
+  return '[$visibleText](${destination?.toString() ?? ''})';
+}

lib/widgets/compose_box.dart

@@ -6,6 +6,7 @@ import 'package:image_picker/image_picker.dart';
 
 import '../api/route/messages.dart';
 import '../model/autocomplete.dart';
+import '../model/compose.dart';
 import '../model/narrow.dart';
 import 'dialog.dart';
 import 'store.dart';
@@ -135,7 +136,7 @@ class ComposeContentController extends ComposeController<ContentValidationError>
   int registerUploadStart(String filename) {
     final tag = _nextUploadTag;
     _nextUploadTag += 1;
-    final placeholder = '[Uploading $filename...]()'; // TODO(i18n)
+    final placeholder = inlineLink('Uploading $filename...', null); // TODO(i18n)
     _uploads[tag] = (filename: filename, placeholder: placeholder);
     notifyListeners(); // _uploads change could affect validationErrors
     value = value.replaced(_insertionIndex(), '$placeholder\n\n');
@@ -157,7 +158,7 @@ class ComposeContentController extends ComposeController<ContentValidationError>
 
     value = value.replaced(
       replacementRange,
-      url == null ? '' : '[$filename](${url.toString()})');
+      url == null ? '' : inlineLink(filename, url));
     _uploads.remove(tag);
     notifyListeners(); // _uploads change could affect validationErrors
   }

test/model/compose_test.dart

@@ -306,4 +306,9 @@ hello
     check(mention(user, silent: false)).equals('@**Full Name|123**');
     check(mention(user, silent: true)).equals('@_**Full Name|123**');
   });
+
+  test('inlineLink', () {
+    check(inlineLink('CZO', Uri.parse('https://chat.zulip.org/'))).equals('[CZO](https://chat.zulip.org/)');
+    check(inlineLink('Uploading file.txt…', null)).equals('[Uploading file.txt…]()');
+  });
 }