Improve fish widget code

[kidonng]

• Remove unused code

• Replace [ with idiomatic test. fish's manpage says:

  The first form (test) is preferred. For compatibility with other shells, the
  second form is available: a matching pair of square brackets ([ [EXPRESSION ] ]).

• Prepend underscore to widget function to avoid name clashing

• Insert command instead of replacing, when not doing smart replace

• Only bind insert mode when not using default keybindings

• Correctly quote variables to support multiline commands

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