Add parsing of workspace.metadata.i18n

[litcc]

minor changes

The current cli only supports reading [package.metadata.i18n], but when there is no package entry in the project root, but there is [workspace.metadata.i18n] data, the available-locales and the available-locales are not read based on the attributes in the workspace. default-locale, resulting in a TODO that only detects en when executing cargo i18n in other languages.

For example, when this is the case:

https://github.com/longbridgeapp/rust-i18n/tree/84ac7dbb598f39901cd5fbc511664ae3cc36b4c1/examples/share-in-workspace

Although it is possible to pass the specified parameters by adding [i18n] directly to Cargo.toml, it is annoying to be prompted with a warning when executing cargo build or other commands!