surface logMessage in extension API

[weinand]

Surface the logMessage property on type Breakpoint.

(for details see #45128)

[weinand]

• I've added a new readonly property "logMessage" to vscode.Breakpoint,
• since there is no way to augment a class with a new constructor signature (Suggestion: Reopen static and instance side of classes TypeScript#2957), I could not add (overloaded) constructors to vscode.proposed.d.ts that take an additional "logMessage" argument.