strings: add Builder.Cap

strings: add Builder.Cap

To report the capacity of the underlying buffer. The method mirrors
bytes.Buffer.Cap.

The method can be useful to know whether or not calling write or grow
methods will result in an allocation, or to know how much memory has
been allocated so far.

Fixes #26269.

Change-Id: I391db45ae825011566b594836991e28135369a78
Reviewed-on: https://go-review.googlesource.com/122835
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>