[lang] support multiline arguments to annotations

**Describe the problem/challenge you have**
I want to be able to annotate a node using an expression formatted over multiple lines, but `ytt` currently only accepts a single line per annotation.

**Describe the solution you'd like**
For example,

```yaml
#@schema/desc """
#@ something something
#@ """
a: 123

#@overlay/match by=overlay.subset({
#@ "kind": "Foo",
#@ "metadata": {"blah": "blah"},
#@ })
b: 123
```

**Anything else you would like to add:**

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