Support installer declarations

[Lordfirespeed]

Does what it says on the tin!

In addition

• Update language version to C# 12
• Remove all default values from the TOML schema definition
• Update DefaultConfig with the aforementioned default values
• Ensure tcli init's created thunderstore.toml uses only configuration from the default config

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 133 lines in your changes missing coverage. Please review.

Project coverage is 5.36%. Comparing base (10b73c8) to head (20e6bcc).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ThunderstoreCLI/Models/ThunderstoreProject.cs	0.00%	40 Missing ⚠️
ThunderstoreCLI/Configuration/Config.cs	0.00%	36 Missing ⚠️
ThunderstoreCLI/Configuration/DefaultConfig.cs	0.00%	19 Missing ⚠️
ThunderstoreCLI/Configuration/ProjectFileConfig.cs	0.00%	15 Missing ⚠️
...hunderstoreCLI/Configuration/CLIParameterConfig.cs	0.00%	8 Missing ⚠️
ThunderstoreCLI/Commands/BuildCommand.cs	0.00%	7 Missing ⚠️
ThunderstoreCLI/PackageManifestV1.cs	0.00%	2 Missing ⚠️
ThunderstoreCLI/API/ApiHelper.cs	0.00%	1 Missing ⚠️
ThunderstoreCLI/Commands/InitCommand.cs	0.00%	1 Missing ⚠️
ThunderstoreCLI/Commands/PublishCommand.cs	0.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #100      +/-   ##
=========================================
- Coverage    5.47%   5.36%   -0.11%     
=========================================
  Files          42      42              
  Lines        2413    2461      +48     
  Branches      237     239       +2     
=========================================
  Hits          132     132              
- Misses       2278    2326      +48     
  Partials        3       3              

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