Create VECTOR type.

[nicktobey]

A vector type is type purpose-built to store a fixed number of floating-point values.

The syntax for some existing engines:

• MariaDB uses the syntax VECTOR(N)and does not allow the user to specify the precision.
  -PGVector uses VECTOR(N) for single-precision (32 bit) floats, but also supports HALFVEC(N) for half-precision (16 bit), and BIT(N) for binary vectors.

• sqlite-vec uses float[N], and also supports int vectors and binary vectors.

• We're committed to supporting the MariaDB syntax, but we may want to add other types for other precisions, like PGVector does.