Fix SUBSTRING with a non-positive start position

[spokodev]

Problem

SUBSTRING / SUBSTR / MID compile SUBSTRING(str, pos, len) to str.substr(pos - 1, len). When pos is 0 or negative, pos - 1 becomes a negative index, and JavaScript's String.prototype.substr() counts a negative start from the end of the string. The result is neither correct nor consistent with any SQL dialect:

alasql("SELECT VALUE SUBSTRING('abcdef', 0, 3)")   // 'f'   (MySQL: '')
alasql("SELECT VALUE SUBSTRING('abcdef', -1, 3)")  // 'ef'  (MySQL: 'f')
alasql("SELECT VALUE SUBSTRING('abcdef', -2, 3)")  // 'def' (MySQL: 'ef')

So a computed start position that lands on 0 or goes negative silently returns wrong characters.

Fix

Follow MySQL semantics (verified against MySQL 8), which alasql already matches for positive positions:

• start = 0 yields an empty string;
• a negative start counts from the end (SUBSTRING('abcdef', -2) -> 'ef'), clamped to an empty string when it reaches past the beginning (SUBSTRING('abcdef', -7) -> '');
• positive start positions are unchanged.

call	before	after (= MySQL 8)
SUBSTRING('abcdef', 0, 3)	'f'	''
SUBSTRING('abcdef', -1, 3)	'ef'	'f'
SUBSTRING('abcdef', -2, 3)	'def'	'ef'
SUBSTRING('abcdef', -2)	'def'	'ef'
SUBSTRING('abcdef', -7, 3)	'bcd'	''
SUBSTRING('abcdef', 2, 3)	'bcd'	'bcd'

Test

Added test/test-substring-negative-start.js. yarn test passes (2193 tests).