Provide intellisense for stored procedures/functions

[isc-tleavitt]

• For stored procedures (INFORMATION_SCHEMA.ROUTINES where ROUTINE_TYPE = 'PROCEDURE'), provide intellisense to support use as TVFs (these should show in a table context and ideally have columns listed)
• For other routines (functions and UDAFs - though not sure off the top of my head how UDAFs are represented in INFORMATION_SCHEMA), provide intellisense to support use in the same context a column of a referenced table would be used.
• For both cases, provide information on the expected arguments

Doing all of this correctly (and perhaps supporting IRIS SQL completely) would likely require changes upstream in vscode-sqltools, at minimum to permit more inversion of control to the DB-specific driver for SQL parser configuration (e.g., to use a custom Hue parser as described in https://docs.gethue.com/developer/development/#sql-parsers rather than just the generic SQL autocomplete parser we're getting from there; this could be a worthy project in itself and probably one best tackled from within InterSystems).

(From comments in #31 )