[SQL] Add default SQL syntax selector command

[deathaxe]

This PR adds a naive command for users to choose default SQL dialect to be assigned to source.sql.

The sql_set_default_syntax basically works for any syntax, but is prefixed sql_ as it is located in and dedicated to SQL syntax package.

Alternatively a general purpose set_default_syntax could be implemented by ST core or Default package.

Ideally, dialect syntaxes could be identified by attributes in Syntax() class, but until this becomes true, rely on a static list of files specified via dialect_file argument to choose from.

FWIW, Right now SQL seems to be the only use case for what is called a "dialect".

[predragnikolic]

If I have a *.sql file open,
and I would like to specify a dialect for that file,
intuitively I would look in the command palette to see if it offers Set Syntax: SQL (MySQL) or Set Syntax: SQL (PostgreSQL).

[deathaxe]

You can do so and even assign *.sql extension to any of MySQL, PostgreSQL or T-SQL.

Syntax names like <Name> (<Base>) denote extended syntaxes based on another. That's not really the case here, because SQL is by default just an alias for MySQL, which can be changed to anything. All dialects are based on SQL (basic) (yes it violates given scheme above).

The point of this PR and provided command is to be able to specify the SQL syntax/dialect to use in syntax highlighted strings of Python, PHP, ... as those don't provide any other way to do so - and just embed source.sql.

That being said, we could hide "SQL" syntax and just assign *.sql extension to MySQL by default - somewhat following the scheme of ShellScript.