Fixes #1079.

Since SE-0001, Swift allows any keyword except inout, var and let as an argument label, and Apple APIs use this a lot (devices(for:), contains(where:), index(of:in:)). SwiftLexer still highlights those labels as keywords:

let d = AVCaptureDevice.devices(for: .audio, noHighlight: 42)

'devices'     Name
'('           Punctuation
'for'         Keyword        <- should look like noHighlight
':'           Punctuation
...
'noHighlight' Name

Fix: a new rule in root, placed before the keyword rules, lexes an identifier that follows ( or , as a Name when it is a label:

• call site: followed by : (f(for: x))
• declaration: followed by a parameter name and : (func f(in x: Int))

The lookahead leaves the rest of the parameter to the existing rules. Excluded from the rule:

• inout, var and let, which SE-0001 doesn't allow as labels. This keeps if let a = x, let b: Int = y and Swift 2 style (inout x: Int) as they are.
• true, false and nil, so dictionary literals like [true: 1, false: 0] keep both keys as Keyword.Constant.
• a lone _, so #selector(foo(_:)) and func f(_ x: Int) are unchanged.

I also compared the output before and after for a ternary in an argument list, case a, b:, default:, generic constraints (<T: P, U: Q>, where T: Equatable, U: Hashable) and multi-line calls. Only the keyword labels change.

Tests: I added tests/snippets/swift/test_keyword_argument_labels.txt, covering the call-site and declaration forms. It fails on master and passes with the fix. The tests/examplefiles/swift/test.swift golden output is unchanged, and the full suite passes (pytest tests --ignore=tests/contrast: 5330 passed, 16 skipped). I skipped tests/contrast because wcag_contrast_ratio isn't installed in my environment.

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