String Indexing

Algo muy util en varios lenguajes es poder acceder a caracteres individuales en cadenas: `"abc"[0]` devuelve `a`, `"abc"[1]` devuelve `b` y `"abc"[2]` devuelve `c`. Valores por encima de eso lanzan error.

The issue names no files or tests; start by locating the interpreter entry point that evaluates string expressions and any existing expression tests. Confirm how character access and out-of-range indices are represented, then verify that the documented examples return the expected characters and larger indices raise an error.

Written by the indexing model from the issue text.