Notice: now we use "lang: " prefix when adding language-related commits

[achimnol]

It has become too cumbersome to find the issue number when committing some changes about a language kernel. I think this adds significant mental overheads now as we have dozens of language kernels with irregularly ordered issue numbers.

So I'd like to make a little more room for flexibility of issue references. Let's just use a simple prefix like "python: ", "rust: ", etc. when committing changes about a specific language kernel.
Of course, still we should use explicit issue numbers for bugs and new feature sets.
There is no clear-cut rule when to use numbers or prefixes – you may even mix them – but let's have two key principles:

1. Make the changes recorded and searchable afterwards.
2. Git itself is the best tool to record changes with good commit messages and atomic commits.

For the record of history, here I gathered all lang-related issues:

• Python Python Support #1
• R R Language Support #2
• PHP PHP Support #4
• NodeJS NodeJS Support #5
• Go Go Language Support #9
• Octave GNU Octave Support #10
• Julia Julia Support #11
• Haskell Haskell support #18
• Git Git support #19
• Lua Lua support #22
• Shell Shell script kernel #31
• TensorFlow + Keras TensorFlow + Keras #32
• PyTorch PyTorch support #43
• C C language support #44
• C++ C++ language support #45
• Java Java language support #46
• Rust Rust language support #48

I left a few issues about languages that are NOT implemented yet, though.