Fix: ignore modeline schema comment in the middle of file after yaml content

[shin19991207]

What does this PR do?

Modeline schema comment should only be supported in the document header comment.

Ignore modeline schema comment if it is in the middle of file after yaml content, e.g.

foo:
  bar
# yaml-language-server: $schema=https://json-schema.org/draft-07/schema#
aa:bbb

or

foo:
  bar
# $schema=https://json-schema.org/draft-07/schema#
aa:bbb

Modeline schema comment in multiline comments at document header is still handled, e.g.

# foo:
#   bar
# yaml-language-server: $schema=https://json-schema.org/draft-07/schema#
aa:bbb

What issues does this PR fix or reference?

Fixes redhat-developer/vscode-yaml#1260

Is it tested? How?

• New automated tests
• Tested manaully

[datho7561]

Looks good to me and works from my testing. Thanks, Morgan!