Add initial tests for first-person pronouns

This commit adds initial first-person tests for first-person
pronouns, which the GitLab style guide prohibits. The test for
'I' is separate from the other tests, because a case-insensitive
test using standard word boundaries will match both 'I/O' (which is
correct usage) and 'i.e.' (which is wrong but for different
reasons).

doc/.linting/vale/styles/gitlab/FirstPerson.yml

+# Checks for use of first person pronouns.
+#
+# For a list of all options, see https://errata-ai.github.io/vale/styles/
+extends: existence
+message: '`%s` is a first-person pronoun. Use second- or third-person pronouns (like we, you, us, one) instead.'
+level: warning
+ignorecase: true
+link: https://docs.gitlab.com/ee/development/documentation/styleguide.html#language
+tokens:
+  - '\bI[ ,;:?!"]|\bI\x27.{1,2}'
+  - me
+  - myself
+  - mine