fix: Don't silently accept multi-table FROM clauses (implicit JOIN syntax)

[alexander-beedie]

Ref: #16662.

• Catch use of old-school implicit JOIN syntax (multiple tables in the FROM clause, constraints in the WHERE) and raise an informative error; currently we require use of explicit JOIN constraints.
• A few minor renames/cleanups, eg: process_set_expr -> process_query.

Note that I do expect to have the implicit form working in the not-too-distant future, but at the moment anyone using this syntax may experience less useful errors or incorrect results, so we need to catch it until it is properly supported.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.01%. Comparing base (69e3824) to head (4bd0442).
Report is 3 commits behind head on main.

Files	Patch %	Lines
crates/polars-sql/src/sql_expr.rs	77.77%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17028   +/-   ##
=======================================
  Coverage   81.00%   81.01%           
=======================================
  Files        1448     1448           
  Lines      190543   190551    +8     
  Branches     2722     2722           
=======================================
+ Hits       154357   154366    +9     
+ Misses      35683    35682    -1     
  Partials      503      503           

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