wasm: emit a clean error for scalar ?T/!T returns instead of an internal ICE

[enghitalo]

Description

On the native wasm backend, a function returning a scalar option/result (e.g. fn f() ?int) aborts the compiler with an internal ICE instead of the intended user-facing error:

$ v -b wasm -o /tmp/x.wasm opt.v
wasm error: get_wasm_type: unreachable type 'int' ast.TypeInfo(ast.UnknownTypeInfo{})

Root cause

In fn_decl (vlib/v/gen/wasm/gen.v), the return-type match rts.info handles option/result only:

• inside the MultiReturn arm (option types are not implemented),
• after the match (result types are not implemented),
• and for the void-option case (returning a void option is forbidden).

A scalar ?T/!T return is not a MultiReturn, so it falls into the else arm and calls get_wasm_type(rt) on the option/result-wrapped type before any guard runs — and that aborts with the unreachable type ... UnknownTypeInfo ICE. The existing guards were effectively dead code for this case.

Fix

Guard option/result in the non-void scalar arm, before lowering the type. g.v_error already exits, so this short-circuits before get_wasm_type.

After

$ v -b wasm -o /tmp/x.wasm opt.v
opt.v:1:8: error: option types are not implemented
    1 | fn f() ?int {
      |        ~~~~

• fn f() !int → result types are not implemented (located).
• fn f() ? (void option) → still returning a void option is forbidden (unchanged).
• Normal returns and the rest of the wasm test suite are unaffected.

This is in the spirit of #27451 (replace internal get_wasm_type ICEs with clean errors), covering the scalar option/result cases specifically.

Tests

Adds vlib/v/gen/wasm/tests/scalar_option_result_error_test.v (asserts the located error is produced and the internal ICE string does not leak). Full vlib/v/gen/wasm/tests/ suite passes; v fmt clean.

Checklist

• tests pass locally (v test vlib/v/gen/wasm/tests/)
• v fmt -w applied