VM: Support Custom Syntax

[schungx]

Introduction

Custom syntax is a unique feature of Rhai, one used by many domain-specific applications to simplify code by providing new syntax suited to the domain usage.

Grain Support

Previously, Grain rejects all custom syntax as fragments. This PR adds support for custom syntax that does not touch the scope.

Custom syntax that touches the scope, like eval, remain non-lowerable. These cases are extremely rare.

Implementation

Grain implements custom syntax via a new Opcode Op::CustomSyntax and a custom_syntax pool.

no_ast

Finally, no_ast works with custom syntax, disabling only the parser half of it, but allows the execution part to work with Grain.

[schungx]

@ImTheSquid Finally got all the CI tests to pass and all the feature combinations properly gated!

Pls take a read thru this... This look like a larger PR but really not so. One new opcode, one new pool, and the relevant implementation.

All other file changes are properly gating the no_ast feature, which used to automatically turn on no_custom_syntax but now needs to co-exist with custom syntax (because Grain can run it).