Allow specification of custom GnuCOBOL configuration files

[emilienlemaire]

[nberth]

Note: if we choose the upstreaming of dialect route, the automated test results will need to be updated.

[nberth]

Other missing bits:

• New version of schemas/superbol-schema-0.1.4.json (with version 0.2.3 I think), with updated definition for dialect option);
• A reference to that new schema in vscode_extension.ml;
• an "Added" entry in CHANGELOG.md.

[nberth]

Just noticed there's one important bit missing: we need to change the description of entry superbol.cobol.dialetct in package.json (so, vscode_extension.ml). Apparently there is no way to have a clean "enum with additional user-provided values", so that needs to become a plain "string". Ideally the list of known dialect names should be listed in the markdown description for the entry.