Extract base class for tests
Rename base intention test method
Add new test for converting heredoc with potential variables
Refactor test case classes:

* Extract new base class
* Specialize test class for PHP language
* Specialize test methods
* Add new - unused by now - test methods

Replicate tests from PhpReplaceDoubleQuotesWithEscapingIntention to PhpNowdocToHeredocIntention
Add test method for checking for unavailable intentions
Widely refactor PhpStringUtil and intention methods using it:

* Create centralized method for looking for string literals on the document
* Use StringLiteralExpression as standard working type
* Process literal textual fragments and escape sequences when walking heredoc contents
* Add new string literal kind checker methods
* Remove duplicate code

Enable conversion from heredoc to nowdoc when found expressions embedded
Make scopes for intentions availability more precise
Improve logic for intention availability preventing ambiguous scenarios between intentions and containing string literals
Create local exceptions for some intentions
Move UI messages to intentions
Add lots of test for new covered scenarios, remove obsolete tests