feat: Add SQL interface support for PostgreSQL dollar-quoted string literals

[alexander-beedie]

A quick coffee-break-sized PR ☕
Adds SQL support for PostgreSQL dollar-quoted string literals¹.

Example

import polars as pl

pl.sql("""
  SELECT
    $$string$$ AS s1,
    $tag$another string$tag$ AS s2
""").collect()

# shape: (1, 2)
# ┌────────┬────────────────┐
# │ s1     ┆ s2             │
# │ ---    ┆ ---            │
# │ str    ┆ str            │
# ╞════════╪════════════════╡
# │ string ┆ another string │
# └────────┴────────────────┘

Footnotes

1. https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-DOLLAR-QUOTING ↩

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.34%. Comparing base (5fc791c) to head (09d5748).
Report is 7 commits behind head on main.

Files	Patch %	Lines
crates/polars-sql/src/sql_expr.rs	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17940      +/-   ##
==========================================
- Coverage   80.49%   80.34%   -0.15%     
==========================================
  Files        1493     1492       -1     
  Lines      196477   196332     -145     
  Branches     2813     2813              
==========================================
- Hits       158155   157745     -410     
- Misses      37801    38066     +265     
  Partials      521      521              

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