Loki: Fix parsing of escaped quotes in LogQL

[svennergr]

What is this feature?

Parsing a visual representation of a query containing escaped quotes caused a bug. This PR fixes parsing of escaped quotes.

Special notes for your reviewer:

1. Open a Loki in code editor with the query:

{app="frontend"} |= "\"line"

2. Change to query builder.
3. Notice that the "line value in the visualrepresentation is now correct - previously it was not.
4. Add any operation.
5. Notice that the resulting query will be something like

{app="frontend"} |= `"line`

[github-actions]

Backend code coverage report for PR #69584
No changes

[github-actions]

Frontend code coverage report for PR #69584

Plugin	Main	PR	Difference
loki	84.98%	84.98%	0%

[grafanabot]

The backport to v10.0.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-69584-to-v10.0.x origin/v10.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x a81cee1d05ade7d09856ff4b7da1b6712382ee3e
# Push it to GitHub
git push --set-upstream origin backport-69584-to-v10.0.x
git switch main
# Remove the local backport branch
git branch -D backport-69584-to-v10.0.x

Then, create a pull request where the base branch is v10.0.x and the compare/head branch is backport-69584-to-v10.0.x.