Add vector_concat() and vector_slice() functions

[dhairyadev26]

This PR adds support for two new vector functions:

• vector_concat(x, y) – Concatenates two vectors of the same type.
• vector_slice(x, start_index, end_index) – Extracts a subvector from the input vector.

These functions were already implemented in Turso, and adding them to libSQL maintains compatibility between the two projects.

Closes #2136

Notes:

• Both functions require vectors of the same type
• vector_slice with negative indices is not supported
• start_index must be less than end_index
• Comprehensive error checking is included for edge cases

A test SQL file is included to verify functionality of both functions.