Improve line/column computations, adhering to LSP standard

* Add helper class TextStepper to facilitate computations
* Reimplement TextUtils methods based on TextStepper
* Adhere to LSP definition of newlines being defined by either "\n", "\r\n" or "\r" on its own.
  Previously, only "\n" was considered.
* Add util method TextUtils.split_lines, allowing usages of Python's splitlines to be replaced

test/solidlsp/test_text_utils.py

@@ -0,0 +1,32 @@
+from solidlsp.ls_utils import TextUtils
+
+
+class TestTextUtils:
+    LINE = "012"
+    TEXT = LINE + "\n" + LINE + "\r\n" + LINE + "\r" + LINE
+
+    def test_split_lines(self):
+        lines = TextUtils.split_lines(self.TEXT, with_ends=False)
+        assert len(lines) == 4
+        for line in lines:
+            assert line == self.LINE
+
+    def test_split_lines_with_ends(self):
+        lines = TextUtils.split_lines(self.TEXT, with_ends=True)
+        assert len(lines) == 4
+        for i, line in enumerate(lines):
+            assert line[: len(self.LINE)] == self.LINE
+        for i, ending in enumerate(["\n", "\r\n", "\r", ""]):
+            assert lines[i][len(self.LINE) :] == ending
+
+    def test_line_col_from_idx(self):
+        assert TextUtils.get_line_col_from_index(self.TEXT, 0) == (0, 0)
+        assert TextUtils.get_line_col_from_index(self.TEXT, 1) == (0, 1)
+        assert TextUtils.get_line_col_from_index(self.TEXT, 3 + 1 + 1) == (1, 1)
+        assert TextUtils.get_line_col_from_index(self.TEXT, 3 + 1 + 3 + 2 + 1) == (2, 1)
+
+    def test_idx_from_line_col(self):
+        assert TextUtils.get_index_from_line_col(self.TEXT, 0, 0) == 0
+        assert TextUtils.get_index_from_line_col(self.TEXT, 0, 1) == 1
+        assert TextUtils.get_index_from_line_col(self.TEXT, 1, 1) == 3 + 1 + 1
+        assert TextUtils.get_index_from_line_col(self.TEXT, 2, 1) == 3 + 1 + 3 + 2 + 1