feat: better ast for encapsedpart

[alexander-akait]

fixes #251

Example:

• "foo $test bar" - syntax: simple and curly: false
• "foo ${test bar}" - syntax: simple and curly: true
• "foo {$test}" - syntax: complex and curly: false

Syntax false for string, because here no syntax 😄

[coveralls]

Coverage increased (+0.1%) to 96.007% when pulling 8842d93 on issue-251 into 32d28f0 on prettier.