Fix additionally inserted whitespace in var(--foo,) and env(--foo,)

[RobinMalfait]

This PR fixes an issue where var(--foo,) with a fallback and env(--foo,) with a fallback injected an extra whitespace character after the comma.

This always happened even if there was already some whitespace. So an input like:

var(--foo, )

Resulted in:

var(--foo,  )

It's using a similar fix as implemented here: 32f1f52#diff-2c849c2fb4295c48ca4d58d85ff61f2d31178ccd99b9190bd7714928a7d8d820

Instead of calling .delim() we check for whitespace being present and manually inject the , and whitespace if necessary.

Test plan

• Added tests for both var() and env() that ensures that no additional whitespace is added after the comma.
• All other tests still pass.

Fixes: #1141