strip last whitespaces in a string template

[nofun97]

Initially, the expression

$"
stuff:
    ${[1, 2, 3] >> $"
        letter:
    "::\i}
"

will output:

stuff:
    letter:

    letter:

    letter:

This is caused by the newline after the word letter: in the string template.

This commit removes the last whitespaces of string expression for better formatting of multiline expression. The output now is:

stuff:
    letter:
    letter:
    letter:

Checklist:

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

[coveralls]

Pull Request Test Coverage Report for Build b1d71291e34a72e597227157958f8aace7ac71a0-PR-587

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.03%) to 48.286%

---

Files with Coverage Reduction	New Missed Lines	%
rel/value_tuple.go	2	75.2%
rel/value_set_dict.go	3	43.36%
<!--	Total:	5		-->

Totals
Change from base Build bcf3599b377a8304eaa4a674fd0561109be04f56:	-0.03%
Covered Lines:	4621
Relevant Lines:	9570

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💛 - Coveralls