Add Cadence language

[srinjoyc]

Description

Added the Cadence programming language which is used by the Flow blockchain for its smart contracts. It already is available via VSCode and the Flow community would love to add it to Github. Cadence is used in every decentralized application on Flow which has over 300+ live projects, a large community (30k+ on discord), and continues to grow in usage both by developers and consumers. Adding this language would make it easier for the community to discovery, browse, and contribute to Flow and Cadence.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com. Yes
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Acdc
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • FlowToken.cdc: https://github.com/onflow/flow-core-contracts/blob/master/contracts/FlowToken.cdc
      • FlowFees.cdc: https://github.com/onflow/flow-core-contracts/blob/master/contracts/FlowFees.cdc
      • get_balance.cdc: https://github.com/onflow/flow-core-contracts/blob/master/transactions/flowToken/scripts/get_balance.cdc
      • mint_tokens.cdc: https://github.com/onflow/flow-core-contracts/blob/master/transactions/flowToken/mint_tokens.cdc
      • setup_account.cdc: https://github.com/onflow/flow-core-contracts/blob/master/transactions/flowToken/setup_account.cdc
      • read_profile.cdc: https://docs.onflow.org/fcl/tutorials/flow-app-quickstart/#querying-the-blockchain
    • Sample license(s): https://github.com/onflow/flow-core-contracts/blob/master/LICENSE
  • I have included a syntax highlighting grammar: https://github.com/onflow/vscode-cadence
  • [ ] I have updated the heuristics to distinguish my language from others using the same extension. NA

Submitting on behalf of the Flow community & core contributors. Tried my best to follow the instructions, let me know if there's anything to fix or improve - thanks!