Prioritise default configurations from alephium.config.ts

[simerplaha]

Towards resolving Default Settings in #291.

This PR ensures that when a build configuration is missing from alephium.config.ts, its default value mentioned in the documentation is used:

• sourceDir will be defaulted to contracts.
• artifactDir is always set to None because currently we do not make any use of it. Issue Implement deployment #84 will decide its fate.
• All compilerOptions are defaulted to false. Although the documentation does not mention this, I'm assuming this is what's expected.

The function that merged compilerOptions from ralph.json and alephium.config.ts is removed. If the file alephium.config.ts is available, the defaults compilerOptions (all false) from alephium.config.ts, defined or not, are used and the ones in ralph.json are overwritten.

This ensures that alephium.config.ts and its defaults are always prioritised over any configuration defined in ralph.json.

A new project containing a *.ral file will still start compilation. But when an empty alephium.config.ts is created, that *.ral file will have to be moved to the contracts directory (alephium.config.ts's default) for compilation.

[tdroxler]

LGTM, I tested it with the alephium template, works fine