Document AppMeta and make it public (#6939)

This PR was made apon reflecting [this message on
Discord](https://discord.com/channels/1059888774789730424/1179936876103225354/1432299089873211443).

The user in this case was trying to figure out whether or not there was
a present mode whenever you access the app meta information. The
documentation doesn't actually confirm if this is the case. It doesn't
list all the values that the run mode can represent.

Hence, I figured adding the `AppMeta` class to our documentation. That
way a user could at least confirm themselves if it is/not supported.

Author: koaning

docs/api/app.md

@@ -9,3 +9,5 @@
 ## AppMeta
 
 ::: marimo.app_meta
+
+::: marimo.AppMeta

marimo/__init__.py

@@ -18,6 +18,7 @@
     # Core API
     "App",
     "Cell",
+    "AppMeta",
     "create_asgi_app",
     "MarimoIslandGenerator",
     "MarimoStopError",
@@ -127,6 +128,7 @@
 from marimo._plugins.stateless.tree import tree
 from marimo._plugins.stateless.video import video
 from marimo._runtime import output
+from marimo._runtime.app_meta import AppMeta
 from marimo._runtime.capture import (
     capture_stderr,
     capture_stdout,

tests/snapshots/api.txt

@@ -1,4 +1,5 @@
 App
+AppMeta
 Cell
 Html
 MarimoIslandGenerator