I2S: add command i2sloop

[Staars]

Description:

New command i2sloop for endless looping of sound file, can work gapless for opus format.
Syntax equal to i2splay, e.g.:
i2sloop2 brown.opus - loops a sound file that could contain brown noise to build your personal noise machine.

The development revealed a bug (memory leak) in the framework, that was introduced some time ago. Using the stack is better anyway in the scope of Tasmota, as we already allocate memory for the task. Thus avoiding multiple allocations later in this task will additionally improve performance a tiny bit.

More small refactoring steps to reduce code duplication.

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.1.250109
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass

[sfromis]

Noticed one detail, not specific to this improvement. That use of index 2 (seems to be for decoder_type) also on the existing I2SPlay command is not mentioned in the docs.