CodeChunk: Accessibility check failures

[nokome]

Over in Encoda we are doing AXE and WCAG2 accessibility checks on generated HTML. There are several failures for CodeChunks (that we are currently skipping). Note some of these are double ups:

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(8) > div > stencila-code-editor > div > menu > select: label: Form elements must have labels (https://dequeuniversity.com/rules/axe/3.5/label?application=axeAPI)"

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(11) > div > stencila-code-editor > div > menu > select: label: Form elements must have labels (https://dequeuniversity.com/rules/axe/3.5/label?application=axeAPI)"

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(8) > stencila-action-menu > nav: landmark-unique: Ensures landmarks are unique (https://dequeuniversity.com/rules/axe/3.5/landmark-unique?application=axeAPI)"

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(11) > div > stencila-code-editor > div > div > div:nth-child(2) > div: scrollable-region-focusable: Ensure that scrollable region has keyboard access (https://dequeuniversity.com/rules/axe/3.5/scrollable-region-focusable?application=axeAPI)"

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(8) > div > stencila-code-editor > div > menu > select: WCAG2AA.Principle4.Guideline4_1.4_1_2.H91.Select.Name: This select element does not have a name available to an accessibility API. Valid names are: label element, title undefined, aria-label undefined, aria-labelledby undefined."

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(8) > div > stencila-code-editor > div > menu > select: WCAG2AA.Principle1.Guideline1_3.1_3_1.F68: This form field should be labelled in some way. Use the label element (either with a \"for\" attribute or wrapped around the form field), or \"title\", \"aria-label\" or \"aria-labelledby\" attributes as appropriate."

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(11) > div > stencila-code-editor > div > menu > select: WCAG2AA.Principle4.Guideline4_1.4_1_2.H91.Select.Name: This select element does not have a name available to an accessibility API. Valid names are: label element, title undefined, aria-label undefined, aria-labelledby undefined."

    Failed: "html > body > main > article > stencila-code-chunk:nth-child(11) > div > stencila-code-editor > div > menu > select: WCAG2AA.Principle1.Guideline1_3.1_3_1.F68: This form field should be labelled in some way. Use the label element (either with a \"for\" attribute or wrapped around the form field), or \"title\", \"aria-label\" or \"aria-labelledby\" attributes as appropriate."