ci: adapt PR size job to add sizes label

[jvbriones]

Adapt PR size job to add sizes label:

default values
size-XS: ≤ 10 lines
size-S: ≤ 100 lines
size-M: ≤ 500 lines
size-L: ≤ 1000 lines
size-XL: > 1000 lines

Also adapt the calculation of changes by adding +0 in the awk END block to ensure that when there are no changes, awk outputs 0 instead of an empty string:

{print add} → {print add+0}
{print del} → {print del+0}

tested here: https://github.com/MetaMask/metamask-mobile/actions/runs/16826192546/job/47663091866

[mcmire]

Seems right to me!