Edit Post: Fix Welcome Guide modal display for Internet Explorer

[aduth]

Fixes #19140

This pull request seeks to resolve a number of issues with the Welcome Guide as it is displayed in Internet Explorer. Most importantly, the modal can now be dismissed.

Before	After

These fixes include:

• Text would overflow in the modal due a Flexbox bug associated with Internet Explorer and the use of align-items: center on Flex containers (reference). Since this style did not appear to have a noticeable impact on the display of the modal, it was simply removed.
• The "Close" button could not be clicked because it relied on a z-index styling to appear "above" the rest of the content in the modal. However, since z-index only applies to positioned elements (reference), it was not taking effect, because the only positioning applied to the element was position: sticky, not supported in Internet Explorer (reference). This was resolved by applying position: relative as a "fallback" positioning, allowing the z-index to take effect as expected.
• The image was intended to appear as vertically centered using absolute positioning. There were no explicit styles used to actually apply the centering effect. While these did not seem to be necessary for other browsers, the explicit centering was introduced to ensure that the image is centered correctly in Internet Explorer.
• The "Finish" button would previously not appear in Internet Explorer, because it was expected to take effect using display: unset, a value unsupported in Internet Explorer (reference). Instead, display: block is used, derived from the effective ("computed") value otherwise expected from display: unset.

This does not include handling to allow the modal background to be clicked to dismiss the guide. I expect this may be a separate issue with the Modal component.

Additionally, I was tempted to remove this bit of styling:

gutenberg/packages/components/src/guide/style.scss

Line 17 in 6f6412a

margin-top: -$header-height;

...since it seems in contradiction with how a modal is expected to be styled, and assumes some vertical centering of the modal contents which otherwise isn't enforced (for Welcome modal, it is applied in the welcome modal styling). Since these were not directly relevant for the Internet Explorer fix, I opted to leave them for separate revision.

Testing Instructions:

Repeat testing instructions from #18041, in Internet Explorer and in your preferred browser.

[gwwar]

As an FYI if folks need a stacking context, any of the other rules on https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Positioning/Understanding_z_index/The_stacking_context will also do the trick, in case changing position has other side effects folks want to avoid.

https://developer.mozilla.org/en-US/docs/Web/CSS/isolation for example only does this, but it's unsupported by IE.

[gwwar]

Thanks @aduth ! This tests well for me, code looks reasonable and works well on IE11 and Chrome.