[GIFs] Restore default alt text (#3893)

* restore default alt text

* factor out gif alt logic + enable require alt text setting

* rm console.log

* don't prefill input + esc handling

* typo

* Nits

* shorten user alt prefix

* Remove unnecessary condition, rename for clarity

* Add comment

---------

Co-authored-by: Dan Abramov <dan.abramov@gmail.com>

src/lib/gif-alt-text.ts

@@ -0,0 +1,36 @@
+// Kind of a hack. We needed some way to distinguish these.
+const USER_ALT_PREFIX = 'Alt: '
+const DEFAULT_ALT_PREFIX = 'ALT: '
+
+export function createGIFDescription(
+  tenorDescription: string,
+  preferredAlt: string = '',
+) {
+  preferredAlt = preferredAlt.trim()
+  if (preferredAlt !== '') {
+    return USER_ALT_PREFIX + preferredAlt
+  } else {
+    return DEFAULT_ALT_PREFIX + tenorDescription
+  }
+}
+
+export function parseAltFromGIFDescription(description: string): {
+  isPreferred: boolean
+  alt: string
+} {
+  if (description.startsWith(USER_ALT_PREFIX)) {
+    return {
+      isPreferred: true,
+      alt: description.replace(USER_ALT_PREFIX, ''),
+    }
+  } else if (description.startsWith(DEFAULT_ALT_PREFIX)) {
+    return {
+      isPreferred: false,
+      alt: description.replace(DEFAULT_ALT_PREFIX, ''),
+    }
+  }
+  return {
+    isPreferred: false,
+    alt: description,
+  }
+}