Add Rhai book.

doc/src/language/modules/export.md

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+Export Variables and Functions from Modules
+==========================================
+
+{{#include ../../links.md}}
+
+A _module_ is a single script (or pre-compiled `AST`) containing global variables and functions.
+
+The `export` statement, which can only be at global level, exposes selected variables as members of a module.
+
+Variables not exported are _private_ and invisible to the outside.
+
+On the other hand, all functions are automatically exported, _unless_ it is explicitly opt-out with the [`private`] prefix.
+
+Functions declared [`private`] are invisible to the outside.
+
+Everything exported from a module is **constant** (**read-only**).
+
+```rust
+// This is a module script.
+
+fn inc(x) { x + 1 }     // script-defined function - default public
+
+private fn foo() {}     // private function - invisible to outside
+
+let private = 123;      // variable not exported - default invisible to outside
+let x = 42;             // this will be exported below
+
+export x;               // the variable 'x' is exported under its own name
+
+export x as answer;     // the variable 'x' is exported under the alias 'answer'
+                        // another script can load this module and access 'x' as 'module::answer'
+```