wasm: add browser stderr builtins

[medvednikov]

Summary

Fixes #27450.

The native wasm browser target could emit calls to eprintln, but the browser-specific wasm builtin only provided println and panic. That made v -b wasm -os browser panic while finalizing the wasm module, even for an empty main.

This adds browser wasm definitions for print, eprint, and eprintln, routing them through the existing __writeln browser host hook, and updates the empty-main regression test so it no longer masks the issue with -no-bounds-checking.

Validation

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew -b wasm -os browser -o "$tmpdir/m.wasm" "$tmpdir/m.v"
• ./vnew -b wasm -os browser -o /tmp/change_color_by_id.wasm examples/wasm/change_color_by_id/change_color_by_id.wasm.v
• ./vnew test vlib/v/gen/wasm/tests/browser_empty_main_test.v
• ./vnew -silent test vlib/v/gen/wasm/tests/

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[chatgpt-codex-connector]

Preserve non-newline print semantics

When the browser host uses the documented/sample __writeln hook as a line-oriented sink (the wasm browser examples implement it with console.log), routing print through the same hook makes print behave like println. Browser wasm code such as print('a'); println('b') will no longer match V's normal output semantics or the existing wasm expectation that the two writes stay on one line, so the non-newline variants should either remain unsupported or use a separate non-line-writing host import.

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