Fix 5 parser tests with LIMIT BY, MySQL @@ vars, and LIKE alias support

[kyleconroy]

Implemented LIMIT BY clause parsing and EXPLAIN output:

• Added LimitBy field to SelectQuery AST
• Capture LIMIT BY expressions instead of skipping them
• Output LIMIT BY in EXPLAIN when no ORDER BY or second LIMIT

Added MySQL @@variable syntax:

• Parse @@variable and @@global.variable as globalVariable function
• Handle @@session.variable variant
• Include original syntax as alias in output

Fixed LIKE expression alias:

• Added Alias field to LikeExpr
• Parse and output alias for LIKE expressions

Updated float formatting:

• Use scientific notation for very small numbers (< 1e-6)
• Strip leading zeros from exponents (e-07 -> e-7)

Enabled tests:

• 00583_limit_by_expressions
• 00590_limit_by_column_removal
• 01337_mysql_global_variables
• 02045_like_function
• 03213_rand_dos