feat(#981): Add bruno methods to allow vars to be deleted and to check for the existence of vars

[trusta]

Description

Solves #981

Add methods to Bru to manipulate env and collection variables :

• clearEnvVars() : Clear all variables from the environment
• hasEnvVar(key) : Check for the existence of a variable in the environment with the specified name
• unsetEnvVar(key) : Remove the specified variable from the environment
• clearVars() : Clear all variables from the collection
• hasVar(key) : Check for the existence of a variable in the collection with the specified name
• unsetVar(key) : Remove the specified variable from the collection

I also added the completion in app :

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