Add helpers for dir-local-variables for 3rd party use

[813gan]

Functions projectile-add-dir-local-variable and projectile-delete-dir-local-variable wraps their built-in counterparts. They always use .dir-locals.el from root of projectile project.

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Thanks!

[813gan]

@bbatsov Could you approve workflow launch?

[bbatsov]

@813gan Seems you'll have to rebase first.

[bbatsov]

@813gan ping :-)

[bbatsov]

I'll merge this as is, as it looks useful and I'll make some tweaks myself.