feat: New tbl_file() and tbl_query() to explicitly access tables and queries as dbplyr lazy tables

[krlmlr]

Related: #38.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.87%. Comparing base (71ff5b1) to head (2de7ff6).

❗ Current head 2de7ff6 differs from pull request most recent head 3b415d8. Consider uploading reports for the commit 3b415d8 to get more accurate results

Files	Patch %	Lines
R/backend-dbplyr__duckdb_connection.R	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   85.94%   85.87%   -0.08%     
==========================================
  Files         106      106              
  Lines        3621     3631      +10     
==========================================
+ Hits         3112     3118       +6     
- Misses        509      513       +4     

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