Add initial support for expressions

[radeksimko]

This PR introduces support for a few expression types as described in more detail in hashicorp/hcl-lang#16

This is a first step and future PRs will bring support for more expression types, so that we can eventually resolve the following:

• Support built-in functions #37
• textDocument/complete: Complete references to named values #38
• no autocomplete on previous resources inputs, attributes etc. #92
• Completion of module variables in module block #93
• Variables (auto suggest/Peak) #191

Examples

[aeschright]

👍

[ghost]

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