recognize .wrl as a vrml export extension

[tylergibbs1]

fixes #740

.wrl is the extension most vrml tools recognize, but cadquery only inferred VRML from .vrml. this maps .wrl to the existing vrml exporter for both shapes and assemblies.

the exporter test now verifies that .vrml and .wrl produce the same vrml header. the assembly save and export cases also infer the format from .wrl instead of passing it explicitly.

validation

• ran the cadquery black fork against all four changed files
• compiled the cadquery and test python sources
• ran git diff check

full pytest was not run locally because this checkout does not have the occt runtime installed.