textDocument/complete: Complete provider names

[radeksimko]

Completing all providers is currently blocked on hashicorp/terraform#24261 but this could be initially implemented in a minimal way where we only complete init'd providers.

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The idea is that after implementing we're able to complete provider names in the label context, e.g.

provider "<HERE>" {
}

Relatedly we may need to decide whether/how to support completion in incomplete declarations, such as

provider "<HERE>

[radeksimko]

Completing incomplete declarations is tracked under #57

[ghost]

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