Change our filters to accept a `mode` parameter instead of a boolean.

[djbe]

Some of our current filters accept a boolean parameter, which isn't always clear "what" that parameter is for:

• camelToSnakeCase: lowercase the components or not
• snakeToCamelCase: trim empty components

For some of our other filters, we already use a "mode" string parameter, that says a bit more about what it does:

• removeNewlines: all or leading
• swiftIdentifier: normal or pretty

Suggestions for modes:

• camelToSnakeCase: lowercase or none
• snakeToCamelCase: trim-empty or none