Add support for source-phase imports (import source / import.source(...))

[guybedford]

Adds support for the TC39 source-phase imports proposal (https://github.com/tc39/proposal-source-phase-imports) for both static and dynamic forms:

// static
import source wasmModule from "./foo.wasm";
import defer * as ns from "./mod.js";

// dynamic
const m = await import.source("./foo.wasm");
const m = await import.defer("./mod.js");

The source phase tells the host to materialize the module's source representation (e.g. a WebAssembly.Module for .wasm) instead of its exports namespace; defer requests a lazily-evaluated namespace. Dropping the phase keyword silently changes runtime semantics, which is what terser was doing on the mozilla-AST round-trip used by tools like Emscripten's acorn-optimizer.

This is the downstream port of upstream terser PR terser#1682 released in 5.48.0.

Static side

• Parser recognises the contextual source / defer phase keyword after import, with peek-based disambiguation so existing patterns like import source from "x" and import defer, {y} from "x" keep parsing as default-name imports.
• AST_Import gains a phase field, null for plain imports.
• mozilla-ast from_moz / to_moz carry phase across, matching the shape produced by acorn-import-phases.
• DEFPRINT(AST_Import) emits the phase keyword between import and the specifier; printing self.phase verbatim handles both source and defer.
• equivalent-to includes phase in shallow_cmp.

Before / after a round-trip through the optimizer:

// before this patch
import wasmModule from "./foo.wasm";        // ! semantics changed

// after this patch
import source wasmModule from "./foo.wasm";

Dynamic side

• New AST_DynamicImport node used to represent a phased dynamic import; carries the phase. Plain dynamic import(x) continues to use the synthetic AST_SymbolRef("import") callee for back-compat with downstream code that already groks that pattern.
• Parser import_meta dispatches on meta / source / defer; import.source / import.defer must be followed by a call, else a parse error is raised, mirroring the proposal.
• mozilla-ast: from_moz ImportExpression builds the AST_DynamicImport callee when M.phase is set; to_moz AST_Call emits a phased ImportExpression { phase } when the callee is AST_DynamicImport.
• DEFPRINT(AST_DynamicImport) prints import.source / import.defer for the callee.
• size, equivalent-to, has_side_effects and may_throw all updated for the new node.

Tests are also included in the backport.

[sbc100]

I'm not too familiar with acorn internals. Do we need/want to land this right away or can we wait for the upstream terser patch to get reviewed?

lgtm assuming upstream thinks its reasonable.

[sbc100]

If you don't want to wait for upstream we can probably still go ahead, just let me know and I'll take a deeper look.

[guybedford]

Will update here after upstream review process rather.

[guybedford]

The upstream PR has now been landed and released and this PR implementation is now based to that updated upstream PR change.

@sbc100 PTAL

[sbc100]

lgtm!

Can you maybe update the description now that the upstream change has landed?

[guybedford]

Sure, updated the description to match upstream.