feat: show placeholder text to explain that ctrl+d closes split

[starpit]

part of #7060

Description of what you did:

My PR is a:

• 💥 Breaking change
• 🐛 Bug fix
• 💅 Enhancement
• 🚀 New feature

Please confirm that your PR fulfills these requirements

• Multiple commits are squashed into one commit.
• The commit message follows Conventional Commits, which allows us to autogenerate release notes; e.g. fix(plugins/plugin-k8s): fixed annoying bugs
• All npm dependencies are pinned.

[codecov]

Codecov Report

Merging #7068 (5bc9652) into master (57c4375) will decrease coverage by 17.71%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #7068       +/-   ##
===========================================
- Coverage   60.07%   42.36%   -17.72%     
===========================================
  Files          86       81        -5     
  Lines        2705     2363      -342     
  Branches      678      618       -60     
===========================================
- Hits         1625     1001      -624     
- Misses        830     1162      +332     
+ Partials      250      200       -50     

Impacted Files	Coverage Δ
packages/core/src/index.ts	0.00% <0.00%> (-100.00%)	⬇️
packages/core/src/util/async.ts	0.00% <0.00%> (-100.00%)	⬇️
packages/core/src/webapp/cancel.ts	0.00% <0.00%> (-100.00%)	⬇️
packages/core/src/webapp/views/toolbar-text.ts	0.00% <0.00%> (-100.00%)	⬇️
packages/core/src/repl/tab-completion.ts	20.00% <0.00%> (-60.00%)	⬇️
packages/core/src/webapp/block.ts	50.00% <0.00%> (-50.00%)	⬇️
packages/core/src/core/symbol-table.ts	50.00% <0.00%> (-50.00%)	⬇️
packages/core/src/core/usage-error.ts	3.31% <0.00%> (-45.59%)	⬇️
packages/core/src/webapp/util/time.ts	5.12% <0.00%> (-43.59%)	⬇️
packages/core/src/repl/split.ts	47.36% <0.00%> (-37.90%)	⬇️
... and 27 more

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