fixes indentation problem from xstr

[nofun97]

initially the expression

$`
abc:
    ${[1, 2, 3] >> $`
        number:
            ${.}
    `}
`

would output

abc:
    number:
    1
    number:
    2
    number:
    3

this commit fixes that, now it outputs

abc:
    number:
        1
    number:
        2
    number:
        3

Checklist:

• [x] Added related tests
• [ ] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build 226cc4ec76b62fa836288d7cec5ab6c224486f13-PR-583

• 56 of 60 (93.33%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.06%) to 48.031%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
syntax/compile_xstr.go	56	60	93.33%
<!--	Total:	56	60	93.33%	-->

Files with Coverage Reduction	New Missed Lines	%
rel/value_set_generic.go	2	74.47%
rel/value_set_str.go	2	55.67%
<!--	Total:	4		-->

Totals
Change from base Build 62cdc725cc5f8bb32fd23f2bac436e350b6b7fb7:	0.06%
Covered Lines:	4575
Relevant Lines:	9525

---

💛 - Coveralls