When pasting multi-line text into a prompt, don't treat the line feeds as "confirm"

[stefanhaller]

This is likely to do bad things; for example, if the prompt is the shell command prompt, then we would run into what looks like a deadlock bug in tcell. In other cases, the characters in the following lines might be treated as random keybindings after the prompt is confirmed; this is very similar to #4234.

In this case, it seems the best we can do is to simply swallow the line feeds. The entire pasted text will then appear as a single long line in the prompt, and hopefully the user knows that they can use ctrl-u to delete it again. If not, they will probably just hit esc to close the prompt.

Fixes #4954.

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for 30169bd1	✅ 14.29%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (30169bd)	Report Missing	Report Missing	Report Missing
Head commit (fd69aa5)	58312	50725	86.99%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4955)	7	1	14.29%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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