Support property-expansion for command

Example:

input.conf

set output TRC to be the same as input.

set target-trc ${video-params/gamma}

ternary operators like.

set target-trc ${?video-params/gamma==pq:hlg}${!video-params/gamma==pq:bt.1886}

evafast like speedup.

SPACE           cycle pause                                                                 #@click
SPACE           no-osd set speed 1; set pause no                                            #@press
SPACE           ignore                                                                      #@release
SPACE           no-osd add speed ${?speed==4.00:0}${!speed==4.00:0.1}; show-text ${speed}   #@repeat

TODO:

[ ] MPV parse the input.conf on startup and shows an error.
[x] Injection via ";" in the string. Split commands by ";" and compare the array length of raw_cmd and expanded. This is probably safe enough.
```
> filename="example; set unsafe true; play another.mp4"
> set title ${title}
> set title example; set unsafe true; play another.mp4
```

natural-harmonia-gropius / input-event

Support property-expansion for command #21