Stop splitting exclude and cleanup globs on commas; prepare 5.0.0

[FeodorFitsner]

Needed for flet-dev/flet#6890 (flet-dev/flet#6839).

Problem

--exclude, --cleanup-app-files and --cleanup-package-files are declared with the args default splitCommas: true, so every value is split on commas, even when the option is repeated:

• A path containing a comma breaks apart. Flet is about to exclude hidden files it discovers automatically, and a hidden file named .a,main.py would become the exclusions .a and main.py, dropping the app's entry point.
• Brace globs don't work: --cleanup-package-files '**/{tests,docs}' becomes the two broken globs **/{tests and docs}.

Change

• splitCommas: false on exclude, cleanup-app-files and cleanup-package-files, matching requirements. Help texts now say each option can be used multiple times.
• arch keeps splitting: its values are fixed ABI names, and Flet passes them comma-joined.
• README: new "Excluding and cleaning up files" section. It also corrects "honoring --exclude globs", since excludes are exact paths.
• All six packages are bumped to 5.0.0, with changelogs, the podspec, the Gradle version and the example lockfiles updated.

Why a major version

This changes the package CLI contract, and the breakage is silent: --exclude build,tests is now one path, so nothing is excluded and there's no error. The Dart API and runtime are unchanged. flet build pins serious_python exactly (serious_python: 4.7.1 in its template), so existing Flet releases are unaffected. Flet switches to repeated flags together with the 5.0.0 pin.

Testing

• dart analyze bin: no issues. package --help shows the new help texts.
• package app -p Emscripten --exclude ".a,main.py" --exclude build --cleanup-app --cleanup-app-files '**/{tests,docs}' on a test app: app.zip contains only main.py and pkg/keep.txt. So .a,main.py and build/ were excluded, main.py was kept, and both tests/ and docs/ were cleaned.