add support for variable-length strings

[ilia-kats]

This uses the vlen-utf8 filter, which is one of the standard filters included with Python-Zarr.

[coveralls]

Pull Request Test Coverage Report for Build 4819715507

• 26 of 30 (86.67%) changed or added relevant lines in 3 files are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.4%) to 85.012%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Filters.jl	22	23	95.65%
src/ZArray.jl	2	3	66.67%
src/metadata.jl	2	4	50.0%
<!--	Total:	26	30	86.67%	-->

Files with Coverage Reduction	New Missed Lines	%
src/ZArray.jl	2	94.29%
src/MaxLengthStrings.jl	5	33.33%
src/metadata.jl	7	78.49%
<!--	Total:	14		-->

Totals
Change from base Build 4698045635:	-1.4%
Covered Lines:	726
Relevant Lines:	854

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 4819746815

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 27 of 32 (84.38%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 85.814%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Filters.jl	22	23	95.65%
src/ZArray.jl	3	5	60.0%
src/metadata.jl	2	4	50.0%
<!--	Total:	27	32	84.38%	-->

Files with Coverage Reduction	New Missed Lines	%
src/MaxLengthStrings.jl	5	33.33%
<!--	Total:	5		-->

Totals
Change from base Build 4698045635:	-0.6%
Covered Lines:	738
Relevant Lines:	860

---

💛 - Coveralls