fix: Fix FormatException caused by '@' in Custom Flutter Version

[franticn]

As described in the documentation on https://fvm.app/documentation/advanced/custom-version, when we want to manage a custom version of the Flutter SDK in FVM, we need to use custom_ as a prefix for the version. However, it does not support naming conventions that include an @ symbol, such as custom_3.7.0@huawei. This is because previously, the FlutterVersion#parse(String version) method would first attempt to parse a FlutterVersion.release type of FlutterVersion, and would split the string at the @ symbol. This would cause parsing errors for custom version numbers like custom_3.7.0@huawei. Therefore, I have prioritized the parsing of FlutterVersion.custom to ensure that such FlutterVersion can be successfully parsed.

[vercel]

@franticn is attempting to deploy a commit to the FlutterTools Team on Vercel.

A member of the Team first needs to authorize it.