…d commits in parsing (#1776)

This change moves the parsing logic to a dedicated
internal/conventionalcommits package and adds support for commit
overrides and nested commits.

- Commit Override: A BEGIN_COMMIT_OVERRIDE/END_COMMIT_OVERRIDE block can
be used to replace the entire commit message with the content inside the
block.
- Nested Commits: BEGIN_NESTED_COMMIT/END_NESTED_COMMIT blocks allow for
multiple conventional commits to be parsed from a single commit message.

Note: an "isNested" field is added for requirement "nested commits
information will only be used for release notes, and not for bumping
semver", see #1769

refer to
[go/librarian:release-please-lite](http://go/librarian:release-please-lite#heading=h.5bformyuaenc)
for discussions on supported/unsupported edge case scenarios.

For #1695

…nd nested commits in parsing (#1776)"

This reverts commit 24b98ee.

…ommits (#1769)

Add a semver package for logic to parse and increase semantic version
according to https://semver.org/, then use it to derive next version
from slices of conventional commits.

Note: according to
[go/librarian:release-command](http://goto.google.com/librarian:release-command)
and [go/librarian:commits](http://goto.google.com/librarian:commits),
nested commit information should only be used for release notes, and not
for bumping semver. This implementation simply disregard nested commit
types when determining version bumps and always do minor for them.


Note: moved conventional_commits.go to a separate package in #1776. 
TODO: Will also rename release_please_lite.go in a separate followup PR.

Additional context:
[go/librarian:release-please-lite](http://goto.google.com/librarian:release-please-lite)


Fixes #1696