Windows: compile plugin with /utf-8 (#354)

[fotiDim]

Summary

• Compiles the universal_ble_plugin target with /utf-8 so MSVC reads Pigeon's generated sources as UTF-8.
• Fixes the clean-build failure reported in [Bug]: [Windows] Build fails with C4819/C2220 because the Pigeon-generated header contains non-ASCII characters and is compiled under a non-UTF-8 system code page #354: C4819 is promoted to C2220 by the /WX that Flutter's apply_standard_settings adds, on systems whose ANSI code page is not UTF-8 (936, 932, 949, 950, ...).

Root cause

windows/src/generated/universal_ble.g.h is UTF-8 without a BOM and contains em dashes copied verbatim from the Dart doc comments in pigeon/universal_ble.dart. Under a DBCS system code page, MSVC decodes those bytes using the ANSI code page and emits C4819; /WX turns it into C2220 and the plugin target fails.

This is not resolved by the Pigeon 29 migration: the current v29.0.10 header still contains the same 8 non-ASCII lines. The flag is generator-version-independent and also covers any future non-ASCII output.

Testing

• flutter analyze — no issues
• flutter test — 285 tests passed
• The existing Windows CI job (flutter build windows) compiles the target. The C4819 failure itself cannot be reproduced on macOS or on the en-US GitHub Windows runner, since those code pages can represent all three UTF-8 bytes.

Fixes #354.