syntax: keep highlighting after spaced XML empty tags

[justadityaraj]

Context

The XML syntax started an attribute identifier on every literal space and then waited for an equals sign. A space before /> therefore opened an identifier region that never ended, causing its highlighting state to carry into following lines.

Changes

• Require a valid XML NameStartChar after spaces or tabs before starting an attribute identifier.
• Add a regression that verifies text after a spaced empty tag does not inherit the identifier style.

User impact

XML documents using the common <tag attr="value" /> form retain correct syntax highlighting on the rest of the document. Attribute names separated with tabs are recognized as well.

Verification

• Regression on clean master — failed with the following line highlighted as identifier.
• go test ./runtime -run TestXMLEmptyTagWithSpace -count=10 — passed.
• go test ./... — passed.
• go generate ./runtime — passed with no tracked output changes.
• go build ./cmd/micro — passed.
• git diff --check — passed.
• go vet ./... — retains the existing unkeyed-literal and copied-lock warnings reproduced on clean master.

Before	After

Fixes #4213