[extension] `azd x init` should generate a `.gitignore` with sensible defaults to improve dev experience

When initializing a new extension, some folders should be ignored by Git. Generating the .gitignore file would save the extension developer some time and improve the experience.

Something like this:

Go

```
bin/
```

Python

```
bin/
build/
generated_proto/
```

Javascript

```
bin/
generated/
node_modules/
```

C Sharp

```
bin/
obj/
```
