feat: add scripts that helps inserting Google-translated content into .po file

[mattwang44]

[WHY]

As a translator, I'd like to have machine-translated content as the base that I can revise, which significantly reduces the translation effort.

[HOW]

Major

• Add .scripts directory, which is expected to be where all translation toolkits lie in. Poetry is set up here.
• Implement script that sends all untranslated entries of the target .po file to Google translation, and inserts translated content into the .po file.

One can run the script with:

.scripts/google_translate.sh library/csv.po

Minor

• Revise the existing from_cn.sh script and move it into .scripts.

One can run the script with:

.scripts/from_cn.sh library/csv.po

[ezio-melotti]

Probably out of the scope of this PR, but eventually I think it might be useful to have a separate translation-tools repo where to put tools that can be shared by different translation projects.
If the tools are in a separate repo, it is also not necessary to create a hidden dir that is not easily discoverable (using _scripts might be a better option, unless it interferes with something).

[josix]

I think we could merge it first, and create another PR for making batch procedure.