These started by converting `self` into a `String`. This lead to crashes when `self` happened to be a Substring, as the input range isn’t necessarily a valid index range in the resulting string in that case.
While I’m here, review related helper functions to improve performance and to avoid more correctness issues.
* rdar://107955097 (FoundationPreview: Batch move string API (continued))
- Move localized uppercase and lowercase to FoundationLocalization
- if-def out of CharacterSet from FoundationPreview. It's not implemented at all there, and having a no-op stub is misleading
* rdar://107955097 (FoundationPreview: Batch move string API (continued))
- Move components separated by string and range of string functions
* rdar://107955097 (FoundationPreview: Batch move string API (continued))
Enable snake case options for JSON encoder and decoder. We haven't been able to enable this option because it needed `CharacterSet`, which hasn't been properly implemented for FoundationPreview. Now that we have `BuiltInUnicodeScalarSet`, which mirrors `CharacterSet`, we can switch to that and enable the options.
* rdar://107955097 (FoundationPreview: Batch move string API (continued))
- Move `StringProtocol.lineRange(for:)` and `paragraphRange(for:)` to FoundationEssentials
- Rename String+Regex.swift to RegexPatternCache.swift
- Consolidate extensions on various String family members and remove one redundant swift file
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Co-authored-by: I-Ting Tina Liu <iting_liu@apple.com>
