desugar: clean up top-level `convert`

Model it on `dquotes` rather than `pack`, since it needs almost zero
state across words.

Manual testing:
  $ echo 'ab %eax' |subx run apps/desugar
  ab 0

1 files changed, 0 insertions, 0 deletions

diff --git a/apps/desugar b/apps/desugar
new file mode 100755
index 00000000..d805ec46
--- /dev/null
+++ b/apps/desugar
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