Add SDC and XDC to TCL language

[nishtahir]

Closes #6092

Description

This adds Synopsis and Xilinx design constraint files as TCL files for syntax highlighting and linguist reporting purposes. I'm not 100% sure if this was everything that needs to be done to support this. I'd appreciate any feedback/guidance anyone may have.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?q=extension%3A.xdc+get_ports+NOT+nothack&type=Code
      • https://github.com/search?q=extension%3A.sdc+get_ports+NOT+nothack&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • sdc
      • xdc
    • Sample license(s):
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

I'm not 100% sure if this was everything that needs to be done to support this.

We have a whole section in the CONTRIBUTING.md file dedicated to this. Read it, you'll see what you're missing 😉 .

Please also update the template in the OP.

[lildude]

You can ignore the CI failure for now. It'll be resolved once #6093 has been merged.

[juj]

@nishtahir thanks for creating a PR on this!