Bring back support for Starlark multi-line strings

## Acceptance Criteria
White space stored in starlark multi-line strings should be preserved.

**Given** a template calling function that returns a multi-line starlark string:
```
#@ def f():
#@ return """ asd1
#@ asd2
#@ asd3
#@ """
#@ end

string: #@ f()
```
when executing `ytt -f .`
**Undesired Result:**
```
string: |
asd1
asd2
asd3
```
**Desired Result:**
```
string: |
asd1
asd2
asd3
```

_Below is the original issue_

---
**What steps did you take? What happened?**
Attempted to use multi-line strings, but they lost their indentation (inexplicably).

```
#@ def f():
#@ return """asd1
#@ asd2
#@ asd3
#@ asd4
#@ """
#@ end

lel: #@ f()

#! generates
#! lel: |2
#! asd1
#! asd2
#! asd3
#! asd4
```

see: https://kubernetes.slack.com/archives/CH8KCCKA5/p1616177776070200

**What did you expect:**

Either:

- the indentation to be preserved. (this is preferred)
- this limitation be clearly documented (at the very least)

```
#@ def f():
#@ return """asd1
#@ asd2
#@ asd3
#@ asd4
#@ """
#@ end

lel: #@ f()

#! to generate
#! lel: |2
#! asd1
#! asd2
#! asd3
#! asd4
```

**Anything else you would like to add:**
Indentation within a multi-line string feature was a conscious trade-off in story #120

No source file or test is named. Start by reproducing the provided Starlark multi-line string example with the ytt CLI and trace where its whitespace is lost; done means the generated YAML preserves the original indentation shown in the desired result.

Written by the indexing model from the issue text.