Add an emergency-command parser to MarlinSerial's RX interrupt.

The parser tries to find and execute M108,M112,M410 before the commands disappear in the RX-buffer.

To avoid false positives for M117, comments and commands followed by filenames (M23, M28, M30, M32, M33) are filtered.

This enables Marlin to receive and react on the Emergency command at all times - regardless of whether the buffers are full or not. It remains to convince hosts to send the commands. To inform the hosts about the new feature a new entry in the M115-report was made. "`EMERGENCY_CODES:M112,M108,M410;`".

The parser is fast. It only ever needs two switch decisions and one assignment of the new state for every character.

One problem remains. If the host has sent an incomplete line before sending an emergency command the emergency command could be omitted when the parser is in `state_IGNORE`.
In that case the host should send "\ncommand\n"

Also introduces M108 to break the waiting for the heaters in M109, M190 and M303.

Rename `cancel_heatup` to `wait_for_heatup` to better see the purpose.

Extend M421 with I and J parameters

Addressing #3325 – Avoid undefined symbol errors in the compiler.

Redo of #3268 by @jbrazio

Following up on #3231

documentation/LCDLanguageFont.md moved to http://www.marlinfirmware.org/ at commit 12d0983757,
but http://www.marlinfirmware.org/ isn't working.

If Marlin is blocking the serial input or command queue for any length
of time (for example more than 2 seconds), it needs to send a message
to serial out to inform the host that it is busy. Marlin should only
send these messages out when busy, and preferably not when trying to
print formatted output.

Addressing #2879. The M115 report should display `MACHINE_UUID` but it
always displays `UUID`.

- Defining `SOURCE_CODE_URL` in `Default_Version.h` overrides it
elsewhere. So don’t define it there.
- Also fix up `language.h` so no `#undef` will be needed.

This is a proposed change. This section of M114 output is a debug
extension according to RepRap wiki.

Keep "astyled" reformatting

Added comment for Czech language (associated with file language_cz.h).

The long descriptor will still be available in M115

Apply `ENABLED` / `DISABLED` macros to language-related files.

and translate them to German.

Now send only temperature for T or B, but not send OK.
Host interprets the line to show the right temperature, but not in
response to M105 then stop and send commands until it ends the autotune.

- Use `setTargetedHotend` in `M200`, as with other commands that use
`T` for the extruder
- Synthesize the "invalid extruder" message, obviating several long
strings