Add CSV plural support

[SkyLucilfer]

Added CSV plural support, as requested by this proposal godotengine/godot-proposals#1291.

Usage:

It functions like how the proposal describes. Some key notes:

• tr_n() now accepts arguments in the form of tr_n(n, message, plural_message = "", context = "").

• Plural translation using CSV should leave out plural_message and context parameters as they aren't used during the translation (which explains the reordering of the parameters).

• How it works: tr_n(n, "KEY") will fetch the correct plural translation from the CSV using adjusted key, i.e. KEY[0], KEY[1] etc. depending on the current locale and n. The system will concatenate the KEY with appropriate subscript for us.

Condition

Because of how the concatenation works, in the CSV users should append [0], [1] and so on for keys mapping plurals for it to work. This is the contract. Examples can be seen in the proposal.

Note for PO users:

Users using PO files to translate will still have to fill in the plural_message field, as PO files expect to have the plural message data (I have documented this in the class reference too). The context parameter is optional.

Test project:

test_project.zip

[dalexeev]

What is the status of this PR?

[timothyqiu]

Putting String(p_src_text) + "[" + String::num_int64(index) + "]" inside various translation classes looks like a hack to me. Since CSV translation uses OptimizedTranslation, I think it's better to implement proper plural handling for that class.