Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?
  This is an update for known file extensions list to support C++'s '.tpp' files.

• What is the current behavior?
  Currently if we include a lib, which contains tpp's, from 'src' folder (within sketch dir) compiler does not copy template files, so the build is failing.

• What is the new behavior?
  .tpp files should be copied on a verify/upload event to build temp folder along with other .cpp/.h etc.

• Does this PR introduce a breaking change, and is
  titled accordingly?
  No, tpps will be processed normally, that should not require any changes in invocation/sketch files.

• Other information:

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