Squashing the 3 commits of #5688:
Commit 1
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Add prefix doc in idris layer
Also remove `idris-ensure-process-and-repl-buffer` as this is not an
interactive command
Add `SPC m l` keybinding for extracting lemma
Bind `SPC m c` to `idris-case-dwim`
Commit 2
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Fix registering idris repl
Commit 3
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Add basic auto-completion for idris mode
