Fix non-converging indentation of adjacent string constants

[edjubert]

What

Fix: adjacent string constants separated by a newline (implicit concatenation) gained one space of indentation on every run, so the output never converged.

Fixes #410.

Before

Formatting the same input three times in a row:

SELECT
    concat('a', x, ' AU '
 'b') AS lib
FROM
    t;

SELECT
    concat('a', x, ' AU '
  'b') AS lib
FROM
    t;

SELECT
    concat('a', x, ' AU '
   'b') AS lib
FROM
    t;

After

SELECT
    concat('a', x, ' AU '
        'b') AS lib
FROM
    t;

Stable from the first run, and the continuation line now follows the current indentation instead of staying at column 1.

Why

tokenize_sql keeps consecutive constants in a single token and preserves the whitespace between them verbatim (the rule commented preserved multiple constants with newline). _add_token then prefixed $sp to every constant of that token without normalizing the whitespace already present. $sp is a single space when the token is not at the start of a line, so each run appended one more space to what the previous run had produced.

The fix rebuilds the separators rather than prefixing them: a single space when the constants are on the same line, the current indentation after a newline. That makes the result independent of the input spacing, which is what idempotency requires.

Tests

• t/test-files/ex85.sql + t/test-files/expected/ex85.sql cover:
  • two constants on consecutive lines in a select list
  • two constants as the last argument of a function call (the nesting case)
  • three constants chained over three lines
  • two constants in a WHERE comparison
• Full suite: prove -l t/ passes (93 tests)
• Output is idempotent, verified over three further runs on the expected file