Rst parser respect :start-after: and :end-before: in include directive (#12972)

* [FEATURE] rst parser respect :start-after: in include

Rst parser now respects `:start-after:` and `:end-before:` attributes
for `include` directive.

* [DOC] include directive parsing proc update

* [TEST] Added unit tests for include rst directive in `rst` module

tests/stdlib/trst.nim

@@ -0,0 +1,79 @@
+# tests for rst module
+
+import ../../lib/packages/docutils/rstgen
+import ../../lib/packages/docutils/rst
+import unittest
+import os
+
+suite "RST include directive":
+  test "Include whole":
+    "other.rst".writeFile("**test1**")
+    let input = ".. include:: other.rst"
+    assert "<strong>test1</strong>" == rstTohtml(input, {}, defaultConfig())
+    removeFile("other.rst")
+
+  test "Include starting from":
+    "other.rst".writeFile("""
+And this should **NOT** be visible in `docs.html`
+OtherStart
+*Visible*
+""")
+
+    let input = """
+.. include:: other.rst
+             :start-after: OtherStart
+"""
+    assert "<em>Visible</em>" == rstTohtml(input, {}, defaultConfig())
+    removeFile("other.rst")
+
+  test "Include everything before":
+    "other.rst".writeFile("""
+*Visible*
+OtherEnd
+And this should **NOT** be visible in `docs.html`
+""")
+
+    let input = """
+.. include:: other.rst
+             :end-before: OtherEnd
+"""
+    assert "<em>Visible</em>" == rstTohtml(input, {}, defaultConfig())
+    removeFile("other.rst")
+
+
+  test "Include everything between":
+    "other.rst".writeFile("""
+And this should **NOT** be visible in `docs.html`
+OtherStart
+*Visible*
+OtherEnd
+And this should **NOT** be visible in `docs.html`
+""")
+
+    let input = """
+.. include:: other.rst
+             :start-after: OtherStart
+             :end-before: OtherEnd
+"""
+    assert "<em>Visible</em>" == rstTohtml(input, {}, defaultConfig())
+    removeFile("other.rst")
+
+
+  test "Ignore premature ending string":
+    "other.rst".writeFile("""
+
+OtherEnd
+And this should **NOT** be visible in `docs.html`
+OtherStart
+*Visible*
+OtherEnd
+And this should **NOT** be visible in `docs.html`
+""")
+
+    let input = """
+.. include:: other.rst
+             :start-after: OtherStart
+             :end-before: OtherEnd
+"""
+    assert "<em>Visible</em>" == rstTohtml(input, {}, defaultConfig())
+    removeFile("other.rst")