feat: add options to substring for start parameter being negative (#508)

The substring function `start` parameter has different behavior between
different
backends when it's value negative. Sqlite will wrap around the end of
the input string,
whereas postgres will start from the left of the input string.

extensions/functions_string.yaml

@@ -53,6 +53,13 @@ scalar_functions:
       A `start` value of 1 refers to the first characters of the string.  When
       `length` is not specified the function will extract a substring starting
       from position `start` and ending at the end of the string.
+
+      The `negative_start` option applies to the `start` parameter. `WRAP_FROM_END` means
+      the index will start from the end of the `input` and move backwards.
+      The last character has an index of -1, the second to last character has an index of -2,
+      and so on. `LEFT_OF_BEGINNING` means the returned substring will start from
+      the left of the first character.  A `start` of -1 will begin 2 characters left of the
+      the `input`, while a `start` of 0 begins 1 character left of the `input`.
     impls:
       - args:
           - value: "varchar<L1>"
@@ -61,6 +68,9 @@ scalar_functions:
             name: "start"
           - value: i32
             name: "length"
+        options:
+          negative_start:
+            values: [ WRAP_FROM_END, LEFT_OF_BEGINNING, ERROR ]
         return: "varchar<L1>"
       - args:
           - value: "string"
@@ -69,6 +79,9 @@ scalar_functions:
             name: "start"
           - value: i32
             name: "length"
+        options:
+          negative_start:
+            values: [ WRAP_FROM_END, LEFT_OF_BEGINNING, ERROR ]
         return: "string"
       - args:
           - value: "fixedchar<l1>"
@@ -77,24 +90,36 @@ scalar_functions:
             name: "start"
           - value: i32
             name: "length"
+        options:
+          negative_start:
+            values: [ WRAP_FROM_END, LEFT_OF_BEGINNING, ERROR ]
         return: "string"
       - args:
           - value: "varchar<L1>"
             name: "input"
           - value: i32
             name: "start"
+        options:
+          negative_start:
+            values: [ WRAP_FROM_END, LEFT_OF_BEGINNING ]
         return: "varchar<L1>"
       - args:
           - value: "string"
             name: "input"
           - value: i32
             name: "start"
+        options:
+          negative_start:
+            values: [ WRAP_FROM_END, LEFT_OF_BEGINNING ]
         return: "string"
       - args:
           - value: "fixedchar<l1>"
             name: "input"
           - value: i32
             name: "start"
+        options:
+          negative_start:
+            values: [ WRAP_FROM_END, LEFT_OF_BEGINNING ]
         return: "string"
   -
     name: regexp_match_substring