feat: Add variables auto completion

[MasterLambaster]

Description

Add autocomplete for environment variables.

https://github.com/usebruno/bruno/assets/134304/701515eb-9e95-4571-89f5-ef71ec33eb5f

The trigger is {{ that shows all available environment variables. Supports all fields that have collection passed to it, i.e auth, headers etc. Might need sanity check for all the places where it's currently applied. Fixes #1141, #606, #412.

Contribution Checklist:

• [x] The pull request only addresses one issue or adds one feature.
• [x] The pull request does not introduce any breaking changes
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[end3rbyte]

I am having a little bug:

You can reproduce with the collection under /bruno/packages/bruno-tests/collection with environment prod

The suggested value "process" shouldn't be here.

Also I don't think, you should display the value of the variable. It will display secrets on the screen, which is not good in terms of security.

[MasterLambaster]

@busy-panda process is an nested object that could be accessed, normally via dot notation. It's only visible if it's actually set. Regarding hiding values: there are couple existing issues for the hiding secret values and it should be done on the global level rather this particular case.

[acuenca-facephi]

Nice job, this should be included in the following releases.