Add vector_concat() and vector_slice() functions

[dhairyadev26]

Description

This PR implements two new vector functions to maintain compatibility with Turso:

1. vector_concat(X, Y) - Concatenates two vectors of the same type
2. vector_slice(X, start_idx, end_idx) - Extracts a subvector from start_idx (inclusive) to end_idx (exclusive)

These functions were already available in Turso but were missing in libSQL, as noted in issue #2136.

Implementation Details

• Added vector_func_impl.c with implementations of both functions
• Included this file in libsql-sqlite3/src/vector.c
• Created comprehensive tests in test_vector_functions.sql
• Added documentation in README-vector-functions.md

Supported Vector Types

Both functions support all vector types except:

• FLOAT1BIT for vector_slice (bit-by-bit extraction is more complex)

Testing

The implementation has been tested with the provided SQL test file that covers:

• Basic functionality for both functions
• Different vector types (FLOAT32, FLOAT64)
• Edge cases (out of bounds indices, negative indices, etc.)

Fixes #2136