Support user definitions in substrings

I'm using jsonnet to generate docker-compose configs. I'd like to pass bazel defines/vars into the generated files.

Maybe something like this:
```py
ext_strs = {
"foo_cmd": " ".join([
"--key0=val0",
"--key1=val1",
"--mode=$(foo_mode)",
]),
"bar_cmd": " ".join([
"--key2=val2",
"--key3=val3",
"--mode=$(bar_mode)",
]),
},
```

Unfortunately, `jsonnet.bzl` only does substitution if the `$(...)` is the entire string.
https://github.com/bazelbuild/rules_jsonnet/blob/12979862ab51358a8a5753f5a4aa0658fec9d4af/jsonnet/jsonnet.bzl#L128

I was thinking of using a `select` statement, but that's broken inside dicts:
https://github.com/bazelbuild/bazel/issues/3902

I can promote variables to top-level:

```py
ext_strs = {
"foo_cmd": " ".join([
"--key0=val0",
"--key1=val1",
]),
"foo_mode": "$(foo_mode)",
"bar_cmd": " ".join([
"--key2=val2",
"--key3=val3",
]),
"bar_mode": "$(bar_mode)",
},
```

But this makes "foo_mode" and "bar_mode" required, since jsonnet doesn't support default values for extVar: https://github.com/google/go-jsonnet/issues/247

This is unpleasant, since we have several `jsonnet_to_json` rules, reusing the same `foo.jsonnet`, enabling & disabling various containers. I'd like to avoid enumerating all custom args for containers that have been explicitly disabled.

The request I'll make here is to add support for `$(...)` within larger strings.

Start in jsonnet/jsonnet.bzl around line 128 and trace how substitutions are handled when a value contains more than a single $(...) expression. Define the expected behavior for embedded user definitions such as $(foo_mode) in larger strings, then inspect the repository's existing rule tests to determine where coverage belongs and verify the generated configuration output.

Written by the indexing model from the issue text.