Use slow connection mode when `navigator.connection.saveData` is enabled

[shuding]

We are already considering cases like 2g network here:

swr/src/config.ts

Line 45 in 69ec9df

['slow-2g', '2g'].indexOf(navigator['connection'].effectiveType) !== -1

just need to consider the data saving mode to the same place.

Ref: https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/save-data

[Tak-Iwamoto]

@shuding
Can I try this?

[shuding]

@Tak-Iwamoto yeah feel free to take it.