[FrameworkBundle] PhpExtractor bugfix and improvements

PhpExtractor currently only handles simple strings which match an overly-specific token sequence.

This change adds support for 
- heredoc / nowdoc
- inconsistent whitespace when parsing
- escaped sequences in strings
- `transChoice`

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |