Fix algorithm to find balanced var() pairs

[Poetro]

Follow-up to PR #112

Fix #114

The parenthesis matching was not checking the right balance before.
For the declaration value var(--box-shadow, 0px 2px 8px 0px rgba(0, 0, 0, 0.5)) it would extract the following parts

{
  "pre": "",
  "body": "--box-shadow, 0px 2px 8px 0px rgba(0, 0, 0, 0.5",
  "post": ")"
}

as it finds the var( look for the closing ) token, which it then finds just after 0.5. Instead it should look for ( ) pairs, and check if the opening ( is after var. This way in the previous case, it would find:

{
  "pre": "",
  "body": "--box-shadow, 0px 2px 8px 0px rgba(0, 0, 0, 0.5)",
  "post": ""
}

[MadLittleMods]

Thanks for the contribution @Poetro! Sorry for the delay 🙇

Appreciate the clear code and tests ❤️