Document that lines are zero-indexed

[connorshea]

In hindsight this should have been obvious, but I got tripped for a little while on the tests being registered with the wrong line number since I assumed line numbers would be 1-indexed because they're 1-indexed in the UI.

It'd be nice to have the 0-indexing documented so I would have known this from the start :)

vscode-test-adapter-api/src/index.ts

Lines 194 to 195 in 5014a25

	/** The line within the specified file where the suite definition starts (if known). */
	line?: number;

(it should also be documented for line on TestInfo as well)