Fix mixed indents in block strings

[janedbal]

• bug fix
• BC break? no

---

• When using custom indents (spaces) and block mode, inner part of block strings were always indented with tab.
• This showed up in our git-based whitespace check in pre-commit (git diff-index --check --cached HEAD -- $FILES) as space before tab in indent error.
• The real usecase is PHPStan baseline of deprecated usages where newline is very often.

Before:

inner:
    msg: '''
    	string
    	with newline
    '''

After this fix:

inner:
    msg: '''
        string
        with newline
    '''

[janedbal]

@dg Any chance you find a minute to look at this? Thank you

[dg]

Wouldn't it be easier to solve it in StringNode using property $indentation as in BlockArrayNode?

[janedbal]

Wouldn't it be easier to solve it in StringNode using property $indentation as in BlockArrayNode?

Tried in #78, but StringNode is also created in Parser::parseValue where I was not sure how to get the indent properly...