Support array declarations

[lolski]

Product change and motivation

Adds support for declaring double arrays. This lets users work with embeddings, and later express vector-similarity queries.

1. Declaring double array

define
attribute embedding,
    value double[64];

2. Inserting an array value:

insert
$x isa document,
    has embedding [0.1, 0.2, 0.3];

Implementation

• Updated Pest grammar to allow array declarations
• AST & parser: BuiltinValueTypeArray struct and visitors
• Updated tests: declaring double array, inserting an array, and calling a function using an array