Add a note of caution about using shared regex in provideTerminalLinks

[eamodio]

Testing #101300

If an extension uses a shared RegExp instance with the global flag in provideTerminalLinks with await calls , it could lead to unexpected results. This is because a RegExp object with global flag maintains state as you loop over the matches, and if there are any await calls inside that loop, another call to provideTerminalLinks could be executed, re-using that same RegExp instance -- effectively mixing/corrupting the results.

Since IMO, RegExp parsing (especially with the global flag) will likely be common within this API, I think it is worth a note of caution.