Add evaluate to point

[PEZ]

What has Changed?

• Add command for evaluating up to cursor, ctrl+alt+enter

• Restructured evaluation code a bit to accept functions that evaluate to the selection and the text, rather than using a lot of boolean params and that if/else hell.
• Introduced a module get-text.ts where some of these functions live
• Also added three variables to custom REPL snippets
  1. $selection
  2. $head - the text between the start of line and the cursor (So the above command can be created with a custom snippets, even if now noone needs to 😄
  3. $tail - the text between the cursor and the end of the list (No idea when this would ever be needed, but symmetry)

Fixes #1057

My Calva PR Checklist

I have:

• Read How to Contribute.
• Directed this pull request at the dev branch. (Or have specific reasons to target some other branch.)
• Made sure I have changed the PR base branch, so that it is not published. (Sorry for the nagging.)
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and either tested it there if so, or mentioned it in the PR.
• Tested the VSIX built from the PR (so, after you've submitted the PR). You'll find the artifacts by clicking Show all checks in the CI section of the PR page, and then Details on the ci/circleci: build test. NB: There is a CircleCI bug that makes the Artifacts hard to find. Please see this issue for workarounds.
  • Tested the particular change
  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
• Referenced the issue I am fixing/addressing in a commit message for the pull request.
  • If I am fixing the issue, I have used GitHub's fixes/closes syntax
• Added to or updated docs in this branch, if appropriate

Ping @PEZ, @kstehn, @cfehse, @bpringe