pgsql: Install errcodes.txt for use by extensions.

Install errcodes.txt for use by extensions.

Maintainers of out-of-tree PLs typically need access to the set of
error codes. To avoid the need to duplicate that information in some
form in PL source trees, provide errcodes.txt as part of a server
installation.

Thomas Munro, based on a suggestion from Andrew Gierth
Discussion: https://postgr.es/m/87woykk7mu.fsf%40news-spur.riddles.org.uk

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1fd8690668635bab9dfa16b2885e6e474f8451ba

Modified Files
--------------
src/backend/Makefile       |  2 ++
src/backend/utils/Makefile | 10 ++++++++++
src/tools/msvc/Install.pm  |  3 +++
3 files changed, 15 insertions(+)