Add more built-in string functions

[dev-dami]

Description

The standard library needs more string manipulation functions.

Suggested Functions

• split(str, delimiter) - Split string into array
• join(arr, delimiter) - Join array into string
• trim(str) - Remove whitespace
• replace(str, old, new) - Replace substrings
• startsWith(str, prefix) / endsWith(str, suffix)
• toUpper(str) / toLower(str)

Where to Start

• pkg/eval/builtins.go - Add builtin function implementations
• docs/builtins.md - Document the new functions

Good First Issue

This is a good issue for newcomers! Each function can be implemented independently, so feel free to pick one and submit a PR.