Add QMK Tap Dance Feature to ESC/Backtick key

[salcode]

In #3 we swapped Esc and Backtick (`), which overall is a win.

However, it does make Esc slightly less accessible (now Fn + Esc is necessary).

I was reading about the QMK Tap Dance feature and if I understand correctly, we could keep the Esc key as Backtick (`) but when the Esc key is double-tapped, it would act as Esc (instead of backtick).

[salcode]

Note: When defining the keycode, e.g.

enum tapdance_keycodes {
    TD_ESCBT,              //Tap Dance Esc / Backtick
};

we can NOT add it to the existing alt_keycodes block

// Do NOT add tapdance keycode here!
enum alt_keycodes {
    U_T_AUTO = SAFE_RANGE, //USB Extra Port Toggle Auto Detect / Always Active
    U_T_AGCR,              //USB Toggle Automatic GCR control
    DBG_TOG,               //DEBUG Toggle On / Off
    DBG_MTRX,              //DEBUG Toggle Matrix Prints
    DBG_KBD,               //DEBUG Toggle Keyboard Prints
    DBG_MOU,               //DEBUG Toggle Mouse Prints
    MD_BOOT,               //Restart into bootloader after hold timeout
};

when I do, I'm seeing error messages like

Compiling: keyboards/massdrop/alt/keymaps/salcode-drop-alt/keymap.c                                 [OK]
Linking: .build/massdrop_alt_salcode-drop-alt.elf                                                   [ERRORS]
 |
 | /usr/local/Cellar/arm-gcc-bin@8/8-2019-q3-update_1/bin/../lib/gcc/arm-none-eabi/8.3.1/../../../../arm-none-eabi/bin/ld: .build/massdrop_alt_salcode-drop-alt.elf section `.relocate' will not fit in region `ram'
 | /usr/local/Cellar/arm-gcc-bin@8/8-2019-q3-update_1/bin/../lib/gcc/arm-none-eabi/8.3.1/../../../../arm-none-eabi/bin/ld: region `ram' overflowed by 679208 bytes
 | collect2: error: ld returned 1 exit status
 |
make[1]: *** [.build/massdrop_alt_salcode-drop-alt.elf] Error 1
make: *** [massdrop/alt:salcode-drop-alt] Error 1
Make finished with errors

for more details see this reddit post [QMK] region `ram' overflowed](https://www.reddit.com/r/olkb/comments/bfj442/qmk_region_ram_overflowed/)