Improve compatibility with engine-specific syntax restrictions (@macro)

[travispaice]

Some database engines utilise @ for restricted keyword functionality, e.g., bigquery system parameters like @@reservation or denoting MS SQL variables like @variableX.

Sqlmesh will incorrectly interpret these as a reference to an internal @macro, and the model fails. There does not appear to be any work around to this limitation currently, other than changing database engines.

It would be great if we could add options to change the default @ sqlmesh macro syntax via the configuration file (and/or per model definition) to an alternate character or string.

[travispaice]

More context for MS SQL issue using variables in a query. Attempting to use the @ syntax causes sqlmesh error "Failed to resolve macros for < model details> .... Macro variable is undefined".

-- Work around T-SQL syntax variable issues by using sqlmesh @DEF
--@DEF(VariableX, 'X');

 DECLARE @VariableX CHAR(1) = 'X';

SELECT
    1 AS ColumnA
   ,@VariableX AS VariableX;

You can work around the MS SQL syntax limitation in this case by using the sqlmesh @def syntax instead, but this may not always be possible.

Also impacts UDF such as:

CREATE OR ALTER FUNCTION dbo.udf_add_two_ints (@IntegerA INT, @IntegerB INT)
RETURNS INT
AS
    BEGIN
        RETURN @IntegerA + @IntegerB;
    END;

@mattiasthalen , more context for you.