add doc/en/scripting/builtins/libsession.md

doc/en/scripting/builtins/libsession.md

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+# *session* Library
+
+This library is used to store data that is not reset when loading/unloading content, and is a recommended alternative to using the `_G` table.
+
+The stored data is not saved when the engine terminates.
+
+To minimize the risk of name collisions, it is recommended to use pack prefixes (`pack_id:table_name`), or, if a pack stores a single table in *session*, use the pack name as the entry name.
+
+```lua
+-- Returns a table by name. Creates an empty table if it does not exist.
+session.get(name: str) -> table
+
+-- Deletes a table by name.
+session.reset(name: str)
+
+-- Returns true if the table has already been created by calling session.get
+session.has(name: str) -> boolean
+```