fix(material/datepicker): calendar aria-descriptions start/end date

[zarend]

For date ranges, add aria-descriptions to the cell of the current start
date and also for end date.

Popuplate aria descriptions with the expected value of the ARIA accessible name of the
matStartDate and matEndDate inputs. Introduces _computeAriaAccessibleName function to
implement ARIA acc-name-1.2 specificiation.

Fixes #23442 and #23445

[zarend]

This is pretty far along, but still a draft. Here's the remaining TODO's to make this ready:

• Fix examples to have both placeholder and aria-label
• rename startdate-arialabelledby to startdate-accessible-name
• update documentation to reflect best practice of giving matStartDate/matEndDate an aria label. (I plan on sending out a separate PR for documentation).
• address 3 failing CI checks

[crisbeto]

LGTM

[zarend]

@crisbeto my thought process here is that I recommend using _computeAriaAccessibleName only as a last resort. In most situations I would expect there to be an easier solution than predicting what the ARIA api will do 😂 . I typed the argument as HTMLInputElement because the unit tests only cover the Datepicker's use case.

tomorrow, I can take a look at HTMLTextareaElement and see what it would take to add that. IIRC text area would be almost identical to input.

[crisbeto]

I was mostly referring to setting the accepted types to HTMLInputElement|HTMLTextareaElement, I don't think that we need to unit test textarea specifically.

[zarend]

Okay, we can do that, we would also need to change this line that detects input element to: if (ssrSafeIsHTMLInputElement(currentNode) || ssrSafeIsTextareaElement(currentNode)) {. That way we properly handle the placeholder attribute, title attribute, and for/id with <textarea/>.

[mmalerba]

LGTM

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