feat: add collection functions, string interpolation, and if()

[alvinreal]

Summary

Adds collection functions, string interpolation, conditional function, and type checking as specified in #24.

New Functions

Collection functions:

• keys(.obj) — Returns array of map keys
• values(.obj) — Returns array of map values
• unique(.arr) — Removes duplicates from array (preserves order)
• first(.arr) / last(.arr) — First/last element (null for empty)
• sum(.arr) — Sum of numeric array (returns int or float)
• group_by(.arr, "field") / groupby() — Group array of objects by field

Conditional:

• if(condition, then_value, else_value) — Ternary conditional

Type checking:

• is_array(.field) — Returns true/false

String Interpolation

Strings can now contain {expr} for inline evaluation:

set .greeting = "Hello, {.name}!"
set .full = "{.first} {.last}"
set .desc = "Total: {.price * .qty}"
set .msg = "Hi, {lower(.name)}!"

• Plain strings without {} remain as regular string literals
• Supports field references, arithmetic, and function calls inside {}
• Escape with \{ for literal braces

Tests (17 integration tests)

• keys_of_map, values_of_map — Map introspection
• unique_array — Deduplication
• first_and_last, first_and_last_empty — Array element access
• sum_integers, sum_floats — Numeric aggregation
• group_by_field — Grouping objects
• if_true_branch, if_false_branch — Conditional
• is_array_true, is_array_false — Type checking
• string_interpolation_basic, string_interpolation_multiple — Basic interpolation
• string_interpolation_with_expression — Arithmetic in interpolation
• string_no_interpolation_plain — Plain strings unaffected
• string_interpolation_with_function — Function calls in interpolation

Fixes #24