Implement import aliasing

[bagoette]

Motivation

When working with C3 modules, I found myself wanting to use shorter or more descriptive names for imported modules. This feature request adds Python/TypeScript-style import aliasing to C3, allowing developers to import modules with custom aliases.

Changes

This PR implements import as syntax that allows aliasing imported modules:

import std::core:dstring as sb;
import std::io as c;

fn void main() 
{
    DString builder = sb::temp_with_capacity(32);
    defer builder.free();
    builder.append("Hello, C3!")
    
    c::printfn(builder.str_view());
}

Key Implementation Details

• New TOKEN_AS keyword: Added to lexer and parser infrastructure
• Extended ImportDecl & Module_ structure: Added alias field to store the alias name
• Enhanced import parsing: Modified parse_import() to handle optional as alias_name syntax
• Updated name resolution: Added sema_find_decl_by_alias() to resolve symbols through aliases
• Error handling: Validates alias names and prevents reserved keywords

Features Supported

• ✅ Basic aliasing: import std::io as console;
• ✅ List imports with aliases: import std::ascii as a, std::io as foo;
• ✅ Mixed aliased and non-aliased imports import std::ascii, std::io as foo;
• ✅ Error messages for invalid alias names
• ✅ Prevention of reserved keyword usage as aliases

Testing

Added tests covering:

• ✅ Basic functionality (import_as_works.c3t)
• ✅ List imports (import_as_list_works.c3t)
• ✅ Error cases for invalid identifiers (import_as_error_ident.c3t)
• ✅ Error cases for reserved keywords (import_as_error_keyword.c3t)
• ✅ Edge cases and boundary conditions

Implementation Notes

• Minimal invasive changes: Only touched necessary files for clean integration
• Backwards compatible: Existing import syntax continues to work unchanged

Concerns

• Seems like C3 is allowing redundant module names and is handling ambiguous declarations so I didn't add a check for redundant import aliases

[lerno]

So, as you've already heard, this is not a thing to be included as it is.

Generally the path shortening in C3 is intended to automatically create aliases for you, rather than having to come up with (inconsistent) aliases (naming is the hardest thing in programming and all of those things). This is why you consistently see io::printn("Hello, world") rather than std::io::printn("Hello, world")

By having this scheme, I also try to encourage library writers to think about their prefixes, to already keep them short and sweet to begin with. Aliasing allows people to get away with much worse module names.

That said the lack of aliasing for modules is kind of a natural thing to add and a missing feature in the cases the library writer was an idiot when doing naming.

So I was thinking of adding something like alias foo = module bar::baz;, but there was no real consensus on the syntax.

The discussion was here: https://discord.com/channels/650345951868747808/820761627967488040/1374730243092381769

[lerno]

So I'm going to close this, but is positive to adding an alias some_path = module foo::bar or something similar. The exact syntax should be worked out, so if you open an issue for it, we'll discuss this there. Keep this code around though, you can probably reuse part of it.