Support ${issueType} in githubIssues.issueBranchTitle

[Copilot]

Orgs that use GitHub issue types want to encode them in branch names (e.g. ${issueType}-${issueNumber}), but variableSubstitution did not recognize ${issueType}, so the placeholder leaked into branch names verbatim.

Changes

• GraphQL: fetch issueType { name } on the IssueBase fragment in queries.gql and queriesExtra.gql. Skipped queriesLimited.gql since older GHES versions may not expose the field.
• Types / parsing: thread issueType through GraphQL.Issue, the parsed Issue interface, and parseGraphQLIssue.
• variableSubstitution: add three new cases — issueType (raw), sanitizedIssueType, and sanitizedLowercaseIssueType — mirroring the existing ${issueTitle} family. When the issue has no type, the placeholder is left as-is, consistent with ${user} when unknown.
• Docs: extend the githubIssues.issueBranchTitle setting description in package.nls.json and the variable list in documentation/IssueFeatures.md.
• Tests: cover each new variable plus the no-issue-type fallback in utils.test.ts.

Example

With an issue typed Production Bug Fix (#7):

// settings.json
"githubIssues.issueBranchTitle": "${sanitizedLowercaseIssueType}-${issueNumber}"
// → production-bug-fix-7

"githubIssues.issueBranchTitle": "${issueType}-${issueNumber}"
// → Production Bug Fix-7   (use sanitized variants for branch-safe output)