Text Editor: Get/Set region text API

Add the ability to get/set the selected text.

**Calling the new methods:**

- `bpy.data.texts["Text"].region_as_string()`
- `bpy.data.texts["Text"].region_from_string("Replacement")`

tests/python/bl_pyapi_text.py

@@ -0,0 +1,65 @@
+# SPDX-License-Identifier: Apache-2.0
+
+# ./blender.bin --background -noaudio --python tests/python/bl_pyapi_text.py -- --verbose
+import bpy
+import unittest
+
+
+class TestText(unittest.TestCase):
+
+    def setUp(self):
+        self.text = bpy.data.texts.new("test_text")
+
+    def tearDown(self):
+        bpy.data.texts.remove(self.text)
+        del self.text
+
+    def test_text_new(self):
+        self.assertEqual(len(bpy.data.texts), 1)
+        self.assertEqual(self.text.name, "test_text")
+        self.assertEqual(self.text.as_string(), "\n")
+
+    def test_text_clear(self):
+        self.text.clear()
+        self.assertEqual(self.text.as_string(), "\n")
+
+    def test_text_fill(self):
+        tmp_text = (
+            "Line 1: Test line 1\n"
+            "Line 2: test line 2\n"
+            "Line 3: test line 3"
+        )
+        self.text.write(tmp_text)
+        self.assertEqual(self.text.as_string(), tmp_text + "\n")
+
+    def test_text_region_as_string(self):
+        tmp_text = (
+            "Line 1: Test line 1\n"
+            "Line 2: test line 2\n"
+            "Line 3: test line 3"
+        )
+        self.text.write(tmp_text)
+        # Get string in the middle of the text.
+        self.assertEqual(self.text.region_as_string(((1, 0), (1, -1))), "Line 2: test line 2")
+        # Big range test.
+        self.assertEqual(self.text.region_as_string(((-10000, -10000), (10000, 10000))), tmp_text)
+
+    def test_text_region_from_string(self):
+        tmp_text = (
+            "Line 1: Test line 1\n"
+            "Line 2: test line 2\n"
+            "Line 3: test line 3"
+        )
+        self.text.write(tmp_text)
+        # Set string in the middle of the text.
+        self.text.region_from_string("line 2", ((1, 0), (1, -1)))
+        self.assertEqual(self.text.as_string(), tmp_text.replace("Line 2: test line 2", "line 2") + "\n")
+        # Large range test.
+        self.text.region_from_string("New Text", ((-10000, -10000), (10000, 10000)))
+        self.assertEqual(self.text.as_string(), "New Text\n")
+
+
+if __name__ == "__main__":
+    import sys
+    sys.argv = [__file__] + (sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else [])
+    unittest.main()