shebangs: fix broken shebangs like #!python

[botantony]

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I tried to fix broken shebang for watchman formula (related issue: Homebrew/homebrew-core#210978) but rewrite_shebang didn't work. This patch allows to fix broken shebangs such as #!python, #!python3, etc.

[botantony]

UPD: think it would be reasonable to made this change for the rest languages that support shebang rewriting

[MikeMcQuaid]

Looks good, thanks @botantony!