Fix: conditional statement parsing edge case

[georgesittas]

This PR aims to fix the following case:

>>> from sqlmesh.core.dialect import parse_one
>>> parse_one("@IF(cond, VACUUM ANALYZE)", dialect="postgres")
'@IF(cond, VACUUM ANALYZE' contains unsupported syntax. Falling back to parsing as a 'Command'.
Alias(
  this=MacroFunc(
    this=Anonymous(
      this=IF,
      expressions=[
        Column(
          this=Identifier(this=cond, quoted=False)),
        Command(this=VACUUM)])),
  alias=Identifier(this=ANALYZE, quoted=False))

The problem is that we're currently producing an Alias node as shown above, which happens because SQLGlot tokenizes both VACUUM and ANALYZE as TokenType.COMMAND, instead of producing a STRING for the latter.

This is a SQLMesh-related edge case, since SQLGlot doesn't generally expect a statement to show within a function, such as @IF in this case.