Remove the `$main` keyword

As we now only expose the main trace in the frontend (the aux trace being derived from buses), we should remove the `$main` keyword used to access the i-th column with the binding `$main[i]`.

It should be removed from the parser, the handling of trace accesses in the MIR, and from the documentation.

_Originally posted by @bobbinth in https://github.com/0xMiden/air-script/pull/438#pullrequestreview-3104259358_

The issue mentions removing the `$main` keyword from the parser, MIR handling, and documentation. Start by locating the parser definition for keywords and trace accesses in the codebase. Look for files related to parsing and MIR (Mid-level IR) transformation. After removing the keyword, update any tests that reference `$main` and ensure the documentation is cleaned up. Verify the change by running the project's test suite.

Scritto dal modello di indicizzazione a partire dal testo della issue.