* Use UTF-8 for multi-byte character I/O everywhare

This fix is required for "libstdc++".
Without this fix, "libstdc++" only works for 7bit ASCII.

It fixes https://github.com/microsoft/pict/issues/24 .

* Use current locale instead of fixed UTF-8 locale

It is good for non-UTF-8 (ANSI/OEM) encoding input.
Use environment variable "LANG" or "LC_CTYPE" to change input encoding.

(example)
$ LC_CTYPE="C.UTF-8" pict utf8-input.txt

* Add document for non-ASCII (UTF-8/ANSI) input encoding

From February 2021, in order to provide feedback on pull requests, Code Scanning workflows must be configured with both `push` and `pull_request` triggers. This is because Code Scanning compares the results from a pull request against the results for the base branch to tell you only what has changed between the two.

Early in the beta period we supported displaying results on pull requests for workflows with only `push` triggers, but have discontinued support as this proved to be less robust.

See https://docs.github.com/en/free-pro-team@latest/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#scanning-pull-requests for more information on how best to configure your Code Scanning workflows.

* Create codeql-analysis.yml

* Rename the action