Add support for loading bundle from fs.FS

[antonlindstrom]

This commit aims to add a method for loading bundles from fs.FS. The
change is done to be able to get a nice way to load bundles from an
embedded file system.

Since this can be done by implementing the exported API and using
ParseMessageFileBytes externally, this is not a change that is a must
have. Rather a nice shorthand for using the fs.FS.

This is how it can be used:

package main

import (
        "embed"

        "github.com/nicksnyder/go-i18n/v2/i18n"
)

//go:embed locale.*.json
var LocaleFS embed.FS

func main() {
        bundle := i18n.NewBundle(language.English)
        bundle.RegisterUnmarshalFunc("json", json.Unmarshal)

        bundle.LoadMessageFileFS(LocaleFS, "locale.sv.json")
}

Thanks!

[hachi8833]

I definitely want the feature❤️

[nicksnyder]

Build checks were broken on master but I just fixed that so could you rebase on master?

I think this will fail the Go 1.9 build. Can you use build tags to make this function available in newer versions of go without breaking older versions of Go?

[codecov]

Codecov Report

Merging #246 (58515e2) into main (5d2ec5f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #246   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files          14       14           
  Lines        1178     1178           
=======================================
  Hits          974      974           
  Misses        141      141           
  Partials       63       63           

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[antonlindstrom]

Thank you so much for the good review! I completely missed the backwards compatibility. I've moved the method to it's own file and added a build tag, tested with both 1.16 and 1.15 (pre io/fs).