Add support for matching word boundaries

[mrkishi]

Currently, values will trigger replacements in the middle of words (see BUILD vs REBUILDING example in the added test case).

While that may be sometimes desirable, there's no way to use delimiters to match word boundaries, as they are escaped before being added to the regex.

This PR adds support for raw \b characters in the delimiters, and makes [ '\b', '\b' ] their default.

Thoughts?

[Rich-Harris]

It should be possible to do this with the pureExternalImports option (which I've just realised is undocumented 😬 — details here)

[Rich-Harris]

ah whoops, replying to wrong thread. ignore my last

[Rich-Harris]

This is a good idea, thanks 👍 There were lots of conflicts after the recent round of merges and I thought it best not to treat \b as a special case, so I took a slightly different route in #18. Will close this