Inline getFreeStack and mention 2.5.0 helper method

[mcspr]

Amending #1195 with new ESP.getFreeContStack
Definition instead of function, -6 lines

[Misiu]

can we add elif here just like in https://github.com/xoseperez/espurna/pull/1208/files#diff-52135e36de9de9ff791c1184a170b897R50, this way for 2.5.0 we won't have that extern "C" and rest.
Sorry if this is a bumb comment, I recently start learning Arduino and stuff.

[mcspr]

much less things on the screen was the main reason.
i am not an expert on c++, just felt like a better approach :) and as it turns out from #1205 all that multiline constructs inside .ino broke compilation when on Windows. moving to header also helped with that

[mcspr]

After reading the source for cont_init and cont_get_free_stack I think getFreeStack() should be renamed to something like getTopUsedStack(), because it does not actually return currently used stack, but max amount used since startup.

Looking at postmortem code, current stack pointer can be retrieved via asm("a1") (and note the check for system / cont stack location). My basic understanding of this - sp/"a1" is "top", so just subtracting it with stack start will get used amount.
https://github.com/esp8266/Arduino/blob/master/cores/esp8266/core_esp8266_postmortem.c#L150-L159
Poking around tasmota code I did find section using it for exactly that:
https://github.com/arendst/Sonoff-Tasmota/blob/e4ebee738b437fcc101f80e2f8d7df119aea9f02/sonoff/xdrv_99_debug.ino#L219-
But I am not sure how this will behave with async libs.