please preserve leading whitespace after the ytt inline-code token

## Overview

* indentation_preserve_request
* ytt version 0.45.0

## Problem

* ytt processing does not preserve leading whitespace for `#@` expressions
* ytt appears to remove all whitespace after the `#@` when evaluating ytt-enhanced YAML

## Example

* consider the following well-formed ytt YAML document
* we use a ytt function to output a variable
* we use ytt `#@` expressions to specify inline python/starlark code inside the function
* we define a multiline string variable `str_multiline`
* we then assign the function result to yaml data output

```
## -------------------------------------------------------------------
## begin_ ytt.functions

#@ def feature_request_preserve_indentation():
#@ str_multiline = ('''
#@ charlie_info:
#@ charlie_one: 111
#@ charlie_two: 222
#@ charlie_three: 333
#@ delta_info:
#@ delta_one: 111
#@ delta_two: 222
#@ delta_three: 333
#@ echo_info:
#@ echo_one: 111
#@ echo_two: 222
#@ echo_three: 333
#@ ''')
#@ return(str_multiline)
#@ end

## -------------------------------------------------------------------
## begin_ yaml data output

dataroot:
demo_string: #@ feature_request_preserve_indentation()
```

## Results

* we do not want the undesirable result BEFORE
* instead, we want the desirable result AFTER

### Non desired result -- BEFORE

```
dataroot:
demo_string: |2

charlie_info:
charlie_one: 111
charlie_two: 222
charlie_three: 333
delta_info:
delta_one: 111
delta_two: 222
delta_three: 333
echo_info:
echo_one: 111
echo_two: 222
echo_three: 333
```

### Yes desired result -- AFTER

```
dataroot:
demo_string: |
charlie_info:
charlie_one: 111
charlie_two: 222
charlie_three: 333
delta_info:
delta_one: 111
delta_two: 222
delta_three: 333
echo_info:
echo_one: 111
echo_two: 222
echo_three: 333
```

## Feature request
* please allow ytt code regions to preserve leading whitespace that appears after the `#@` token

## Rationale
* sometimes whitespace is significant (as in this example where the `#@` token is used for a multiline string variable
* the current parsing strategy seems to assume that leading whitespace after the `#@` token is not relevant
* this assumption (if correct) limits the kind of code one can deploy using ytt

---
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Reproduce the supplied ytt 0.45.0 example and compare the current output with the requested output. Start by tracing handling of the `#@` inline-code token and leading whitespace in ytt's parser or evaluator; done means the multiline string preserves the nested indentation shown in the AFTER example without changing unrelated expressions.

Written by the indexing model from the issue text.