feat(ui): support shift+tab for going to the previous tab

[XXMA16]

Description of change

Implemented Shift+Tab for scrolling a tab back Closes #63

Concerns

• Changed the if else's in HexdumpCommand#parse to a match
• ShiftTab -> Back takes up a lot of space and might be a weird name
• CancelPrevious doesn't really need a check_sub since Hexdump is not the first tab

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 32 lines in your changes missing coverage. Please review.

Project coverage is 3.32%. Comparing base (aba6112) to head (2df403d).

Files with missing lines	Patch %	Lines
src/tui/command.rs	0.00%	16 Missing :warning:
src/tui/state.rs	0.00%	16 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #70 +/- ## ======================================== - Coverage 3.34% 3.32% -0.01% ======================================== Files 19 19 Lines 2399 2412 +13 ======================================== Hits 80 80 - Misses 2319 2332 +13 ```

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[XXMA16]

Should I change ShiftTab to ⇧+Tab?

[orhun]

Hmm, that's actually a good idea :)