[accessibility] Add scope="col" attribute to first and second column headers.

[jooyoungseo]

For all the column headers denoted with <th> tag, we need to add scope = "col" attribute to explicitly make a semantic binding for accessibility improvements.

Please take care of this issue after resolving #679 so that only column headers are denoted with <th scope = "col" ...> tag excepting title and subtitle.

Relevant issue (#678) talks about row headers, which have to be denoted with <th scope = "row" ...>.

[jooyoungseo]

This also applies to gt::tab_stubhead(). Stub head also has to include scope = "col" attribute.