Add public imports

Co-authored-by: Copilot <copilot@github.com>

specs/IMPORTS.md

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+# Imports
+The import statement allows one module access to the public declarations of another module.
+
+## Syntax
+The import statement uses the following syntax:
+
+```c2
+import module_name;
+```
+
+They can optionally be prefixed by the `public` keyword, in which case the module will
+export everything in the import transitively.
+
+For instance,
+
+```c2
+--- a.c2
+module a;
+import b;
+
+--- b.c2
+module b;
+public import c;
+
+--- c.c2
+module c;
+
+// Some declarations
+```
+
+In this example, both module a and b can access the declarations in module c.