Allow parentheses in validation for value strings

[roelnoort]

Hi,

I made this pull request to solve the issue in described in:
vercel/serve#649

In short:

1. The serve library can be used to set up security headers.
2. One new header, the Permissions-Policy has a changed syntax which is currently rejected because it fails the schema validation imposed by this repository.
3. The schema validation does not allow parentheses () to be used. I added this by changing this regex pattern:

"^[a-zA-Z0-9_!#$%&'*+.;/:, =^`|~-]+$"

to this pattern:

"^[a-zA-Z0-9_!#$%&()'*+.;/:, =^`|~-]+$"

Also, I noticed that, after cloning and running npm install, I could not run npm test as yarn was not being installed. Therefore I added yarn as a development dependency to package.json.

Best, Roel

[paulshryock]

Is anything blocking this from being merged?