Adds writers for remaining constraints

[popematt]

Issue #, if available:

None

Description of changes:

• Adds writers for AnnotationsV2, Contains, Element, FieldNames, Ieee754Float, and Regex constraints.

Related PRs in ion-schema, ion-schema-tests, ion-schema-schemas:

None.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Attention: 46 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
...hema/writer/internal/constraints/ContainsWriter.kt	0.00% <0.00%> (ø)
.../writer/internal/constraints/Ieee754FloatWriter.kt	0.00% <0.00%> (ø)
...iter/internal/constraints/TimestampOffsetWriter.kt	0.00% <0.00%> (ø)
...chema/writer/internal/constraints/ElementWriter.kt	40.00% <40.00%> (ø)
...nschema/writer/internal/constraints/RegexWriter.kt	0.00% <0.00%> (ø)
...ma/writer/internal/constraints/FieldNamesWriter.kt	0.00% <0.00%> (ø)
...writer/internal/constraints/AnnotationsV2Writer.kt	0.00% <0.00%> (ø)

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