Add localStorage commands and documentation for get_item function (#23)

This pull request adds support for reading values from local storage in
addition to the existing functionality for writing and removing items.
The primary change is the introduction of a new `get_item` command and
its documentation.

**Local Storage Enhancements:**

* Added a new `get_item` command to the storage schema, allowing
retrieval of a value by key from local storage, returning an empty
string if the key does not exist.
* Documented the new `get_item` function in `docs/api/storage.md`,
including usage and return value details.

---------

Co-authored-by: io-eric <io-eric@users.noreply.github.com>

Author: io-eric

docs/api/storage.md

@@ -18,6 +18,15 @@ Saves a key-value pair to local storage.
 void set_item(webcc::string_view key, webcc::string_view value);
 ```
 
+### `get_item`
+
+Reads a value from local storage by its key. Returns an empty string if the key
+does not exist.
+
+```cpp
+webcc::string get_item(webcc::string_view key);
+```
+
 ### `remove_item`
 
 Removes an item from local storage by its key.

schema.def

@@ -147,6 +147,7 @@ system|command|INIT_VISIBILITY_CHANGE|init_visibility_change||{ document.addEven
 # STORAGE (LocalStorage)
 # ------------------------------------------------------------------------------
 storage|command|SET_ITEM|set_item|string:key string:value|{ localStorage.setItem(key, value); }
+storage|command|GET_ITEM|get_item|string:key RET:string|{ const ret = localStorage.getItem(key) || ""; }
 storage|command|REMOVE_ITEM|remove_item|string:key|{ localStorage.removeItem(key); }
 storage|command|CLEAR|clear||{ localStorage.clear(); }