[#2118] Implement keyword snippets for control-flow keywords (if/for/while/etc.)

[DPrakashh]

This PR implements snippet completions for control-flow keywords (if, for, while, guard, switch, repeat) when they are used in code completion.

The solution involved updating "CodeCompletionSession.swift" to identify keyword completions (.keyword) and overwrite their "insertText" and "textEdit" with the correct LSP snippet strings, setting the insertTextFormat to .snippet.

This ensures that keywords now contain placeholders (e.g., ${1:condition} and $0) required by LSP clients.

Verification
New unit tests were added to "SwiftCompletionSnippetTests.swift" to explicitly verify:

1. The "insertTextFormat" is correctly set to ".snippet".
2. The presence of the required snippet placeholders (e.g., $1, $0).

All local tests, including the focused test (swift test --filter SwiftCompletionSnippetTests/testKeywordIfProvidesSnippet), passed successfully.

Checklist (Based on Contributing Guide)
1-Code has been formatted using `swift format -ipr
2-New tests were added to cover the feature.

Fixes #2118

[DPrakashh]

Hi @ahoppen , @bnbarham , @hamishknight , @rintaro

Hope you're having a good week.

I wanted to check on the status of my PR from four days ago. I know these core completion changes take time to review carefully.
Just confirming that all local tests passed on my end. Would it be possible to get the CI tests triggered when a maintainer is able to look at it?

Thanks for your time!

[ahoppen]

Thank you. The overall structure looks good, I have left a few comments inline.

Also, do you have a screen recording or something like that how this looks like in VS Code? Ie. point VS Code to a locally built version of SourceKit-LSP as described in https://github.com/swiftlang/sourcekit-lsp/blob/main/CONTRIBUTING.md#using-a-locally-built-sourcekit-lsp-in-an-editor.

[DPrakashh]

@ahoppen on it sir

[DPrakashh]

@ahoppen

I have updated the PR to address all previous feedback. Here is a summary of the changes:

1-Dynamic Indentation: Implemented indentationUnitString() to use inferred indentation (spaces or tabs) instead of hardcoded tab characters.
2-Refactored Completion Logic: Simplified CodeCompletionSession to calculate textEdit once, using either the standard text or the keyword snippet as appropriate.

Test Improvements:
1-Added the required copyright header to SwiftCompletionSnippetTests.swift.
2-Updated test assertions to use try XCTUnwrap for better error handling and readability.
3-Added new test cases to verify snippet behavior with both inferred spaces and inferred tabs.

Verification: The attached video demonstrates:
1-Running swift test --filter SwiftCompletionSnippetTests with all tests passing.
2-The if, while, and for snippets expanding correctly in VS Code.
3-The snippets dynamically adjusting their indentation when switching the editor from Spaces to Tabs.

Thank you for the guidance on this! As this is my first contribution to a project of this scale, I appreciate your patience and would be happy to make further adjustments if needed.

final.1.mp4