T2 one namespace generation

[niden]

Hello!

• Type: new feature
• Link to issue: [NFR]: Ability to generate only one namespace #2

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Contracts\Formatter::format() takes a Selection in place of the filter string
GenerateCommand::execute() takes a Selection
Pruning is skipped for any narrowed run, not only a filtered one
--namespace= on generate: one namespace and everything beneath it, root implied
Selection, carrying what a run narrows to
NamespaceNotFound, thrown when a requested namespace matches nothing

Thanks

[github-actions]

Code Metrics Report

	master (c1a4807)	#4 (54ecc89)	+/-
Coverage	98.4%	99.1%	+0.7%
Code to Test Ratio	1:0.9	1:0.9	+0.0
Test Execution Time	1s	1s	0s

Details

  |                     | master (c1a4807) | #4 (54ecc89) |  +/-  |
  |---------------------|------------------|--------------|-------|
+ | Coverage            |            98.4% |        99.1% | +0.7% |
  |   Files             |               57 |           62 |    +5 |
  |   Lines             |             1384 |         1453 |   +69 |
+ |   Covered           |             1362 |         1441 |   +79 |
+ | Code to Test Ratio  |            1:0.9 |        1:0.9 |  +0.0 |
  |   Code              |             2895 |         3119 |  +224 |
+ |   Test              |             2667 |         2925 |  +258 |
  | Test Execution Time |               1s |           1s |    0s |

Code coverage of files in pull request scope (97.6% → 98.9%)

Files	Coverage	+/-	Status
src/Cli/GenerateCommand.php	100.0%	+7.1%	modified
src/Contracts/Formatter.php	0.0%	0.0%	modified
src/Exceptions/MissingAsset.php	100.0%	+100.0%	affected
src/Exceptions/NamespaceNotFound.php	100.0%	+100.0%	added
src/Formatter/JsonFormatter.php	100.0%	+4.7%	modified
src/Formatter/Markdown/Classes.php	0.0%	0.0%	added
src/Formatter/Markdown/Html.php	100.0%	0.0%	modified
src/Formatter/Markdown/Naming.php	100.0%	0.0%	modified
src/Formatter/Markdown/Presentation.php	100.0%	+100.0%	added
src/Formatter/Markdown/Signature.php	100.0%	0.0%	modified
src/Formatter/MarkdownFormatter.php	99.5%	+0.0%	modified
src/Model/Imports.php	100.0%	+8.3%	affected
src/Reader/Docblock.php	100.0%	0.0%	modified
src/Reader/Notation.php	100.0%	+100.0%	added
src/Reader/Php/TypeRenderer.php	100.0%	0.0%	modified
src/Reader/Php/ValueRenderer.php	100.0%	0.0%	modified
src/Reader/PhpReader.php	98.5%	-0.5%	modified
src/Reader/ZephirReader.php	97.4%	+0.5%	modified
src/Selection.php	100.0%	+100.0%	added

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Reported by octocov