started writing lexer after making string lib

string/docs/modify/substring.md

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+# string_Substring
+Get a substring of a string.
+
+## Arguments
+- str (string): the string to get a substring of.
+- start (int): starting index of the substring.
+- end (int): ending index of the substring.
+
+## Returns
+substring (string): the substring of the given string.
+
+## Raises
+- `AllocFail`: raised if Ground failed to allocate memory for the new string.
+- `EndBeforeStart`: raised if the `end` index is less than the `start` index.
+- `OutOfBounds`: raised if either the `end` or `start` index is outside the bounds of the string.
+
+## Example
+### Ground
+```python
+call !string_Substring "Hello, World!" 1 3 &sub
+println $sub # "ell"
+```
+
+### Solstice
+```c
+puts string_Substring("Hello, World!", 1, 3) // "ell"
+```