What problem does this feature solve?

As docs said: inject is equivalent of esbuild's inject option, but it's inconsistent.

Reproduction from esbuild docs: https://repl.rolldown.rs/#eNptUUFugzAQ/MrKF4hEnDtSeskTeiw9OGZJnBob2UvTCPnvXQNNoygSArwzOzvemUQn6kmgo3CTFPO/E/X/uRKaj9q76C1K60/lELzGGKW+tuVm0zimoKgpjJgqsYJbBrfxbHp5uUu+ghZ1iwQreri274zBHsoN7N+gKBqHP4MPBNMzR0UoHswUkBYrnbIxewne2tZfnWT3nTk93O4Fsjgx/Tqqxc44PMwwJOiC76H4a2NPd1dMVKOdv/eGcmocgHHDSDUskrJHUnIuVbDbAZ0ROmMxQkSLmrCF422uHkci74AfVPoMpI5ZjIJysfOhr2EWz/IX7qvho5C7F6stPjMtVfnN8/xAhjNsXMqJ5aCU/lInZKbnjawJPdaWhczDGtHigK5Fpw3GRrCHxEpZiJW+c4KKMJJIvxMuyYw=

relate issue rolldown/tsdown#108